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/userdb/userdbd.c

@@ -47,7 +47,7 @@ static int run(int argc, char *argv[]) {
         if (r < 0)
                 return log_error_errno(r, "Failed to start up daemon: %m");
 
-        notify_stop = notify_start(NOTIFY_READY, NOTIFY_STOPPING);
+        notify_stop = notify_start(NOTIFY_READY_MESSAGE, NOTIFY_STOPPING_MESSAGE);
 
         r = sd_event_loop(m->event);
         if (r < 0)