If there is an attempt to switch from a process-owned VT to a closed VT,
then vt(4) first requests the process to release its VT and only then
realizes that the target VT is closed and, so, the switch is not
possible. So, the driver does not actually do any switch, but at the
same time the owning process is not notified about that and it does not
re-acquire the VT.
This change adds an early check for the target VT state, so that the
switch can be refused before the process coordination dance.
On top of that, the code now checks for a failure of vt_window_switch()
and calls vt_window_postswitch() for the current VT if it is in the
process mode.