Make sure myport is set correctly when running with Port = 0.

Setting the Port configuration variable to zero can be used to make tinc
listen on a system-assigned port. Unfortunately, in this scenario myport
will be zero, which means that tinc won't transmit its actual UDP
listening port to other nodes. This breaks UDP hole punching and local
discovery.

src/net_setup.c

@@ -1037,7 +1037,7 @@ static bool setup_myself(void) {
 
 	/* If no Port option was specified, set myport to the port used by the first listening socket. */
 
-	if(!port_specified) {
+	if(!port_specified || atoi(myport) == 0) {
 		sockaddr_t sa;
 		socklen_t salen = sizeof sa;
 		if(!getsockname(listen_socket[0].udp.fd, &sa.sa, &salen)) {