Pass along and use the actual port we're listening on for peers.exchange.

src/ssb.h

@@ -364,6 +364,13 @@ void tf_ssb_connect_str(tf_ssb_t* ssb, const char* address);
 */
 int tf_ssb_server_open(tf_ssb_t* ssb, int port);
 
+/**
+** Determine the port that a server is listening on.
+** @param ssb The SSB instance.
+** @return The port number, or 0 if not bound.
+*/
+int tf_ssb_server_get_port(tf_ssb_t* ssb);
+
 /**
 ** Stop listening for SHS connections.
 ** @param ssb The SSB instance.