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).
(cherry picked from commit 50fb97935d)

src/login/logind-session.c

@@ -710,6 +710,8 @@ int session_stop(Session *s) {
         if (s->started)
                 session_send_signal(s, false);
 
+        s->started = false;
+
         if (s->seat) {
                 if (s->seat->active == s)
                         seat_set_active(s->seat, NULL);
@@ -721,8 +723,6 @@ int session_stop(Session *s) {
         user_send_changed(s->user, "Sessions\0");
         user_save(s->user);
 
-        s->started = false;
-
         return r;
 }