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().
udev: path_id - fix by-path link generation for scm devices

Set some_transport = true to prevent scm devices from being ignored.

Suggested-by: Harald Hoyer <harald@redhat.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
NEWS: add some clarifications

As suggested by Colin Guthrie on the ML.