login: Queue session for garbage collection on leader death

This ensures sessions are cleaned up properly in case the user service
manager was manually stopped.

src/login/logind-session.c

@@ -96,6 +96,8 @@ static int session_dispatch_leader_pidfd(sd_event_source *es, int fd, uint32_t r
 
         session_stop(s, /* force= */ false);
 
+        session_add_to_gc_queue(s);
+
         return 1;
 }
 
@@ -1272,6 +1274,8 @@ static int session_dispatch_fifo(sd_event_source *es, int fd, uint32_t revents,
         session_remove_fifo(s);
         session_stop(s, /* force = */ false);
 
+        session_add_to_gc_queue(s);
+
         return 1;
 }