daemon-util: Rename starting/stopping message constants

Currently, NOTIFY_READY from daemon-util.h conflicts with NOTIFY_READY
from NotifyState from service.h so let's rename the constants to avoid
the conflict.

src/shared/bus-util.c

@@ -154,7 +154,7 @@ int bus_event_loop_with_idle(
 
                         /* Inform the service manager that we are going down, so that it will queue all
                          * further start requests, instead of assuming we are still running. */
-                        (void) sd_notify(false, NOTIFY_STOPPING);
+                        (void) sd_notify(false, NOTIFY_STOPPING_MESSAGE);
 
                         r = bus_async_unregister_and_exit(e, bus, name);
                         if (r < 0)