nspawn: improve log message on bad incoming sd_notify() message

It's the PID that is wrong, not the UID/GID, be precise.

(cherry picked from commit 95116bd)
(cherry picked from commit 95c20d0)
(cherry picked from commit 5b6e914)
(cherry picked from commit b2d59a9)

Author: poettering

src/nspawn/nspawn.c

@@ -4287,7 +4287,7 @@ static int nspawn_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t r
 
         ucred = CMSG_FIND_DATA(&msghdr, SOL_SOCKET, SCM_CREDENTIALS, struct ucred);
         if (!ucred || ucred->pid != inner_child_pid) {
-                log_debug("Received notify message without valid credentials. Ignoring.");
+                log_debug("Received notify message from process that is not the payload's PID 1. Ignoring.");
                 return 0;
         }