upnpstun.c: TEST: Require root user

New version of /sbin/iptables binary prints nonsense error message when is
called by ordinary non-root user:

  iptables v1.8.2 (nf_tables): unknown option "--dport"

Under root user it works correctly and understands --dport argument.

/sbin/iptables binary obviously does not work without root user, so rather
print error message as debugging why /sbin/iptables printed that nonsense
error message about unknown option.

miniupnpd/upnpstun.c

@@ -576,6 +576,11 @@ int main(int argc, char *argv[])
 		return 1;
 	}
 
+	if (geteuid() != 0) {
+		printf("You need to run this application as root\n");
+		return 1;
+	}
+
 	if (argc == 2)
 		argv[2] = "0";