Fix tftp service restarts

The file /var/run/grml-tftpd.pid is owned by nobody:nogroup,
start-stop-daemon then complains about this when trying to stop the
service.

Fixes:

| start-stop-daemon: matching only on non-root pidfile /var/run/grml-tftpd.pid is insecure

grml-terminalserver

@@ -230,7 +230,7 @@ function createTftpConf
 
 function stopTftp
 {
-  start-stop-daemon --stop --quiet -p "$TFTPD_PID_"
+  start-stop-daemon --stop --quiet -p "$TFTPD_PID_" --user nobody
 }
 function startTftp
 {