logind: allow switching to unused VTs via SwitchTo()

If compositors use the new SwitchTo() logic to map F1-F12, we should allow
them to switch to unregistered VTs, too. Otherwise, the auto-spawn logic
of gettys won't trigger.

Reported-by: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>