logind: avoid creating stale session state files

There were old session state files accumulating in /run/systemd/session.
They confused e.g. "reboot", which thought there were still users logged
in. The files got created like this:

session_stop(Session *s) ->
        ...
        unlink(s->state_file);
        ...
        seat_set_active(s->seat, NULL) ->
                session_save(...);  /* re-creates the state file we just
                                       unlinked */

Fix it simply by clearing the s->started flag earlier to prevent
any further writes of the state file (session_save() checks the flag).