login: share VT-signal handler between sessions

login: share VT-signal handler between sessions

sd-event does not allow multiple handlers for a single signal. However,
logind sets up signal handlers for each session with VT_PROCESS set (that
is, it has an active controller). Therefore, registering multiple such
controllers will fail.

Lets make the VT-handler global, as it's mostly trivial, anyway. This way,
the sessions don't have to take care of that and we can simply acknowledge
all VT-switch requests as we always did.