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/socket-activate/socket-activate.c

@@ -478,7 +478,7 @@ static int run(int argc, char **argv) {
                 if (r < 0)
                         return r;
 
-                notify = notify_start(NOTIFY_READY, NOTIFY_STOPPING);
+                notify = notify_start(NOTIFY_READY_MESSAGE, NOTIFY_STOPPING_MESSAGE);
         }
 
         for (;;) {