udev: skip several operations if already in stopping state

Especially, ignore SIGHUP in such case. Otherwise, RELOADING and READY
notification may be sent to PID1, and PID1 warns about the spurious
state transition.

Prompted by 1aedc8fe5e.

src/udev/udev-manager.c

@@ -247,6 +247,9 @@ int manager_reset_kill_workers_timer(Manager *manager) {
 void manager_exit(Manager *manager) {
         assert(manager);
 
+        if (manager->exit)
+                return;
+
         manager->exit = true;
 
         (void) sd_notify(/* unset_environment= */ false, NOTIFY_STOPPING_MESSAGE);
@@ -277,6 +280,9 @@ void notify_ready(Manager *manager) {
 
         assert(manager);
 
+        if (manager->exit)
+                return;
+
         r = sd_notifyf(/* unset_environment= */ false,
                        "READY=1\n"
                        "STATUS=Processing with %u children at max", manager->config.children_max);
@@ -292,6 +298,9 @@ void manager_reload(Manager *manager, bool force) {
 
         assert(manager);
 
+        if (manager->exit)
+                return;
+
         assert_se(sd_event_now(manager->event, CLOCK_MONOTONIC, &now_usec) >= 0);
         if (!force && now_usec < usec_add(manager->last_usec, 3 * USEC_PER_SEC))
                 /* check for changed config, every 3 seconds at most */