logind: prefer new sessions over older ones on VT switches

logind: prefer new sessions over older ones on VT switches

Our seat->positions[] array keeps track of the 'preferred' session on a
VT. The only situation this is used, is to select the session to activate
when a VT is activated. In the normal case, there's only one session per
VT so the selection is trivial.

Older greeters, however, implement take-overs when they start sessions on
the same VT that the greeter ran on. We recently limited such take-overs
to VTs where a greeter is running on, to force people to never share VTs
in new code that is written.

For legacy reasons, we need to be compatible to old greeters, though.
Hence, we allow those greeters to implement take-over. In such take-overs,
however, we should really make sure that the new sessions gets preferred
over the old one under all circumstances. Hence, make sure we override
the previous preferred session with a new session.