udevd: configure a child process name for worker processes

This effectively reverts commit ff86c92e30,
and re-apply 49f3ee7e74.

The change was dropped due to the process name was not correctly logged,
but the issue was fixed by dd15e4cb57.
Let's set the child process name again.

src/udev/udevd.c

@@ -807,7 +807,7 @@ static int worker_spawn(Manager *manager, Event *event) {
         if (r < 0)
                 return log_error_errno(r, "Worker: Failed to enable receiving of device: %m");
 
-        r = safe_fork(NULL, FORK_DEATHSIG, &pid);
+        r = safe_fork("(udev-worker)", FORK_DEATHSIG, &pid);
         if (r < 0) {
                 event->state = EVENT_QUEUED;
                 return log_error_errno(r, "Failed to fork() worker: %m");