logind: If all user sessions are in closing state, set the overall status to closing.

logind: If all user sessions are in closing state, set the overall status to closing.

PulseAudio for example will keep a client connection open provided
at least one session exists. However, if all sessions are currently
in the process of closing, we should flag that as the overall state
appropriately to better reflect what is happening.

Although this does better reflect the status for any given user, it does
not actually solve the overall problem of PulseAudio still finding some
sessions active and thus not exiting and therefore actually preventing
the session from closing. Future commits will extend sd-login to cope
with this situation.