mxqd: Unlock but do not delete lock file in reaper

The reaper inherits the locked lock file and needs to unlock it. Use
mx_funlock_nodelete instead of mx_funlock so that the lock file is not
removed.

mxqd.c

@@ -1390,7 +1390,7 @@ unsigned long start_job(struct mxq_group_list *glist)
         job->host_pid = getpid();
 
         mx_log_debug("starting reaper process.");
-        mx_funlock(server->flock);
+        mx_funlock_nodelete(server->flock);
         server->flock = NULL;
         mx_mysql_finish(&server->mysql);