logind: handle closing sessions over daemon restarts

logind: handle closing sessions over daemon restarts

It may happen that you have several sessions with the same VT:

 - Open a session c1 which leaves some processes around, and log out. The
   session will stay in State=closing and become Active=no.
 - Log back in on the same VT, get a new session "c2" which is State=active and
   Active=yes.

When restarting logind after that, the first session that matches the current
VT becomes Active=yes, which will be c1; c2 thus is Active=no and does not get
the usual polkit/device ACL privileges.

Restore the "closing" state in session_load(), to avoid treating all restored
sessions as State=active. In seat_active_vt_changed(), prefer active sessions
over closing ones if more than one session matches the current VT.

Finally, fix the confusing comment in session_load() and explain it a bit
better.

https://launchpad.net/bugs/1415104