logind: add session controllers

A session usually has only a single compositor or other application that
controls graphics and input devices on it. To avoid multiple applications
from hijacking each other's devices or even using the devices in parallel,
we add session controllers.

A session controller is an application that manages a session. Specific
API calls may be limited to controllers to avoid others from getting
unprivileged access to restricted resources. A session becomes a
controller by calling the RequestControl() dbus API call. It can drop it
via ReleaseControl().

logind tracks bus-names to release the controller once an application
closes the bus. We use the new bus-name tracking to do that. Note that
during ReleaseControl() we need to check whether some other session also
tracks the name before we remove it from the bus-name tracking list.

Currently, we only allow one controller at a time. However, the public API
does not enforce this restriction. So if it makes sense, we can allow
multiple controllers in parallel later. Or we can add a "scope" parameter,
which allows a different controller for graphics-devices, sound-devices
and whatever you want.
Note that currently you get -EBUSY if there is already a controller. You
can force the RequestControl() call (root-only) to drop the current
controller and recover the session during an emergency. To recover a seat,
this is not needed, though. You can simply create a new session or
force-activate it.

To become a session controller, a dbus caller must either be root or the
same user as the user of the session. This allows us to run a session
compositor as user and we no longer need any CAP_SYS_ADMIN.
logind: make Session.Activate() lazy

Currently, Activate() calls chvt(), which does an ioctl(VT_ACTIVATE) and
immediately calls seat_set_active(). However, VTs are allowed to prevent
being deactivated. Therefore, logind cannot be sure the VT_ACTIVATE call
was actually successful.

Furthermore, compositors often need to clean up their devices before they
acknowledge the VT switch. The immediate call to seat_set_active() may
modify underlying ACLs, though. Thus, some compositors may fail cleaning
up their stuff. Moreover, the compositor being switched to (if listening
to logind instead of VTs) will not be able to activate its devices if the
old VT still has them active.

We could simply add an VT_WAITACTIVE call, which blocks until the given VT
is active. However, this can block forever if the compositor hangs.

So to fix this, we make Activate() lazy. That is, it only schedules a
session-switch but does not wait for it to complete. The caller can no
longer rely on it being immediate. Instead, a caller is required to wait
for the PropertiesChanged signal and read the "Active" field.

We could make Activate() wait asynchronously for the session-switch to
complete and then send the return-message afterwards. However, this would
add a lot of state-tracking with no real gain:
 1) Sessions normally don't care whether Activate() was actually
    successful as they currently _must_ wait for the VT activation to do
    anything for real.
 2) Error messages for failed session switches can be printed by logind
    instead of the session issuing Activate().
 3) Sessions that require synchronous Activate() calls can simply issue
    the call and then wait for "Active" properties to change. This also
    allows them to implement their own timeout.

This change prepares for multi-session on seats without VTs. Forced VT
switches are always bad as compositors cannot perform any cleanup. This
isn't strictly required, but may lead to loss of information and ambiguous
error messages.
So for multi-session on seats without VTs, we must wait for the current
session to clean-up before finalizing the session-switch. This requires
Activate() to be lazy as we cannot block here.

Note that we can always implement a timeout which allows us to guarantee
the session switch to happen. Nevertheless, this calls for a lazy
Activate().
logind: introduce session-devices

A session-device is a device that is bound to a seat and used by a
session-controller to run the session. This currently includes DRM, fbdev
and evdev devices. A session-device can be created via RequestDevice() on
the dbus API of the session. You can drop it via ReleaseDevice() again.
Once the session is destroyed or you drop control of the session, all
session-devices are automatically destroyed.

Session devices mimic the session "active" state. A device can be
active/running or inactive/paused. Whenever a session is not the active
session, no session-device of it can be active. That is, if a session is
not in foreground, all session-devices are paused.
Whenever a session becomes active, all devices are resumed/activated by
logind. If it fails, a device may stay paused.

With every session-device you request, you also get a file-descriptor
back. logind keeps a copy of this fd and uses kernel specific calls to
pause/resume the file-descriptors. For example, a DRM fd is muted
by logind as long as a given session is not active. Hence, the fd of the
application is also muted. Once the session gets active, logind unmutes
the fd and the application will get DRM access again.
This, however, requires kernel support. DRM devices provide DRM-Master for
synchronization, evdev devices have EVIOCREVOKE (pending on
linux-input-ML). fbdev devices do not provide such synchronization methods
(and never will).
Note that for evdev devices, we call EVIOCREVOKE once a session gets
inactive. However, this cannot be undone (the fd is still valid but mostly
unusable). So we reopen a new fd once the session is activated and send it
together with the ResumeDevice() signal.

With this infrastructure in place, compositors can now run without
CAP_SYS_ADMIN (that is, without being root). They use RequestControl() to
acquire a session and listen for devices via udev_monitor. For every
device-node they want to open, they call RequestDevice() on logind. This
returns a fd which they can use now. They no longer have to open the
devices themselves or call any privileged ioctls. This is all done by
logind.
Session-switches are still bound to VTs. Hence, compositors will get
notified via the usual VT mechanisms and can cleanup their state. Once the
VT switch is acknowledged as usual, logind will get notified via sysfs and
pause the old-session's devices and resume the devices of the new session.

To allow using this infrastructure with systems without VTs, we provide
notification signals. logind sends PauseDevice("force") dbus signals to
the current session controller for every device that it pauses. And it
sends ResumeDevice signals for every device that it resumes. For
seats with VTs this is sent _after_ the VT switch is acknowledged. Because
the compositor already acknowledged that it cleaned-up all devices.
However, for seats without VTs, this is used to notify the active
compositor that the session is about to be deactivated. That is, logind
sends PauseDevice("force") for each active device and then performs the
session-switch. The session-switch changes the "Active" property of the
session which can be monitored by the compositor. The new session is
activated and the ResumeDevice events are sent.

For seats without VTs, this is a forced session-switch. As this is not
backwards-compatible (xserver actually crashes, weston drops the related
devices, ..) we also provide an acknowledged session-switch. Note that
this is never used for sessions with VTs. You use the acknowledged
VT-switch on these seats.

An acknowledged session switch sends PauseDevice("pause") instead of
PauseDevice("force") to the active session. It schedules a short timeout
and waits for the session to acknowledge each of them with
PauseDeviceComplete(). Once all are acknowledged, or the session ran out
of time, a PauseDevice("force") is sent for all remaining active devices
and the session switch is performed.
Note that this is only partially implemented, yet, as we don't allow
multi-session without VTs, yet. A follow up commit will hook it up and
implemented the acknowledgements+timeout.

The implementation is quite simple. We use device-nodes exclusively to
identify devices on the bus. On RequestDevice() we retrieve the
udev_device from the device-node and search for an existing "Device"
object. If no exists, we create it. This guarantees us that we are
notified whenever the device changes seats or is removed.

We create a new SessionDevice object and link it to the related Session
and Device. Session->devices is a hashtable to lookup SessionDevice
objects via device-node. Device->session_devices is a linked list so we
can release all linked session-devices once a device vanishes.

Now we only have to hook this up in seat_set_active() so we correctly
change device states during session-switches. As mentioned earlier, these
are forced state-changes as VTs are currently used exclusively for
multi-session implementations.

Everything else are hooks to release all session-devices once the
controller changes or a session is closed or removed.
logind: rename vtconsole to seat0

The seat->vtconsole member always points to the default seat seat0. Even
if VTs are disabled, it's used as default seat. Therefore, rename it to
seat0 to correctly state what it is.

This also changes the seat files in /run from IS_VTCONSOLE to IS_SEAT0. It
wasn't used by any code, yet, so this seems fine.

While we are at it, we also remove every "if (s->vtconsole)" as this
pointer is always valid!
logind: fix seat_can_tty() to check for VTs

A seat provides text-logins if it has VTs. This is always limited to seat0
so the seat_is_seat0() check is correct. However, if VTs are disabled, no
seat provides text-logins so we also need to check for the console-fd.

This was previously:
  return seat_is_vtconsole();
It looked right, but was functionally equivalent to seat_is_seat0(). The
rename of this helper made it more obvious that it is missing the VT test.
logind: fix session_activate(vtnr = 0)

VT numbers start with 1. If a session has vtnr == 0, we must not assume it
is running on a VT.
Note that this could trigger the assert() below as CreateSession() sets
vtnr to 0, not <0.
logind: extract has_vts() from can_multi_session()

We currently use seat_can_multi_session() to test for two things:
 * whether the seat can handle session-switching
 * whether the seat has VTs

As both are currently logically equivalent, we didn't care. However, we
want to allow session-switching on seats without VTs, so split this helper
into:
 * seat_can_multi_session(): whether session-switching is supported
 * seat_has_vts(): whether the seat has VTs

Note that only one seat on a system can have VTs. There is only one set of
them. We automatically assign them to seat0 as usual.

With this patch in place, we can easily add new session-switching/tracking
methods without breaking any VT code as it is now protected by has_vts(),
no longer by can_multi_session().
logind: implement generic multi-session

This enables the multi-session capability for seats that don't have VTs.
For legacy seats with VTs, everything stays the same. However, all other
seats now also get the multi-session capability.

The only feature that was missing was session-switching. As logind can
force a session-switch and signal that via the "Active" property, we only
need a way to allow synchronized/delayed session switches. Compositors
need to cleanup some devices before acknowledging the session switch.
Therefore, we use the session-devices to give compositors a chance to
block a session-switch until they cleaned everything up.

If you activate a session on a seat without VTs, we send a PauseDevice
signal to the active session for every active device. Only once the
session acknowledged all these with a PauseDeviceComplete() call, we
perform the final session switch.

One important note is that delayed session-switching is meant for
backwards compatibility. New compositors or other sessions should really
try to deal correctly with forced session switches! They only need to
handle EACCES/EPERM from syscalls and treat them as "PauseDevice" signal.

Following logind patches will add a timeout to session-switches which
forces the switch if the active session does not react in a timely
fashion. Moreover, explicit ForceActivate() calls might also be supported.
Hence, sessions must not crash if their devices get paused.