uvtd: vt: implement ioctl dummies

Add dummies for the ioctl VT callbacks. Also implement the KD-MODE and
KB-MODE state tracking as it is fairly trivial.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
uvtd: seat: implement session IDs

Session IDs provide a unique ID for each registered session. They are used
to sort the sessions so next/prev return the correct neighbour-sessions.
Furthermore, they can be used by outside users to refer to a session
directly without knowing the session implementation.
Sessions with ID=0 are always linked at the end and considered to be
"unnamed".

The algorithm to switch to a session with a given ID is:
  Try to find the first session with the exact same ID. If there is none,
  return the session with the next higher ID. If there is none, return the
  ID'th session in the list. If there is none, return the last session.

This provides a fairly predictable way of switching between session. It is
modeled after the classic VT F1-F12 keys that switch between sessions. If
a session is not given, these keys will switch to the next higher session
instead.
All "unnamed" sessions are put at the end. So if you have only F1-F4, then
F5-F12 will map to unnamed sessions.

Please note that new sessions are always linked at the end of their group.
So new unnamed sessions are at the far end, new named sessions are linked
in the sorted list but behind all sessions with the same ID.
Hence, the caller should avoid multiple sessions with the same ID,
otherwise, some sessions might not be reachable even though they're named
(unless you use next/prev of course).

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
uvtd: vt: implement VT_GETMODE/SETMODE ioctl state-tracking

These ioctls are used to retrieve and set the VT-mode. That is, the
signals and PID information that correspond to the VT controlling process.
It is notified whenever the VT gets active/inactive so it can react to it
or prevent/acknowledge it.

This doesn't implement the signal-sending logic, yet. It only implements
state-tracking.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
uvtd: vt: return -ENODEV if session died

We now keep a link to our parent seat and set it to NULL when our session
is unregistered. In this case, any further request that depends on the
session being registered and probably a valid seat pointer, we will stop
with ENODEV.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
build: fix binary-link with ld.gold linker

The ld.gold linker doesn't provide --format=default but needs --format=elf
instead. However, this doesn't work with ld.bfd. To avoid any linker
detection, we now link any binary file via partial-linking into a proper
object file itself and then link this object file in the final linking
step.
This also produces a fake libtool *.lo file so the libtool linking command
doesn't complain about PIC/non-PIC problems.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>