network: refuse reloading if already stopping or restarting

Otherwise, if reloading is requested after SIGTERM/SIGINT is received,
a new operation may be started, e.g. creating a new netdev or so,
and that will easily fail and may cause spurious network state.

Also, send STOPPING notification to the service manager when
received SIGTERM/SIGINT.

src/network/networkd-manager.c

@@ -515,6 +515,8 @@ static int manager_stop(Manager *manager, ManagerState state) {
                 assert_not_reached();
         }
 
+        (void) sd_notify(/* unset_environment= */ false, NOTIFY_STOPPING_MESSAGE);
+
         manager->state = state;
 
         Link *link;
@@ -1273,6 +1275,9 @@ int manager_reload(Manager *m, sd_bus_message *message, sd_varlink *varlink, boo
         assert(m);
         assert(!message || !varlink); /* D-Bus and Varlink callers are mutually exclusive */
 
+        if (m->state != MANAGER_RUNNING)
+                return -ESHUTDOWN; /* Refuse reloading if we are stopping or restarting. */
+
         log_debug("Reloading...");
         (void) notify_reloading();