net: fix GetListenPort() to derive the proper port

`GetListenPort()` uses a simple logic: "if `-port=P` is given, then we
must be listening on `P`, otherwise we must be listening on `8333`".
This is however not true if `-bind=` has been provided with `:port` part
or if `-whitebind=` has been provided. Thus, extend `GetListenPort()` to
return the port from `-bind=` or `-whitebind=`, if any.

Fixes https://github.com/bitcoin/bitcoin/issues/20184 (cases 1. 2. 3. 5.)

src/net_processing.cpp

@@ -2711,6 +2711,10 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
                     LogPrint(BCLog::NET, "ProcessMessages: advertising address %s\n", addr.ToString());
                     PushAddress(*peer, addr, insecure_rand);
                 } else if (IsPeerAddrLocalGood(&pfrom)) {
+                    // Override just the address with whatever the peer sees us as.
+                    // Leave the port in addr as it was returned by GetLocalAddress()
+                    // above, as this is an outbound connection and the peer cannot
+                    // observe our listening port.
                     addr.SetIP(addrMe);
                     LogPrint(BCLog::NET, "ProcessMessages: advertising address %s\n", addr.ToString());
                     PushAddress(*peer, addr, insecure_rand);