Add protection against UDP inetd looping attacks

This patch adds protection against a common inetd attack vector wherein
the reply port to UDP inetd services is forged to an internal inetd
service port.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

service.c

@@ -590,6 +590,7 @@ recreate:
 	if (plugin) {
 		/* Internal plugin provides this service */
 		svc->inetd.cmd = plugin->inetd.cmd;
+		svc->inetd.builtin = 1;
 	} else {
 		strlcpy(svc->args[i++], cmd, sizeof(svc->args[0]));
 		while ((cmd = strtok(NULL, " ")))