uvtd: vt: implement VT_GETMODE/SETMODE ioctl state-tracking

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>