logind: allow sessions to share a VT if it's a greeter

Old gdm and lightdm start the user-session during login before they
destroy the greeter-session. Therefore, the user-session will take over
the VT from the greeter. We recently prevented this by never allowing
multiple sessions on the same VT. Fix this now, by explicitly allowing
this if the owning session is a GREETER.

Note that gdm no longer behaves like this. Instead, due to wayland, they
always use a different VT for each session. All other login-managers are
highly encouraged to destroy the greeter-session _before_ starting the
user-session. We now work around this, but this will probably not last
forever (and will already have nasty side-effects on the greeter-session).