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.)

test/functional/test_runner.py

@@ -276,6 +276,7 @@ BASE_SCRIPTS = [
     'feature_minchainwork.py',
     'rpc_estimatefee.py',
     'rpc_getblockstats.py',
+    'feature_bind_port_externalip.py',
     'wallet_create_tx.py --legacy-wallet',
     'wallet_send.py --legacy-wallet',
     'wallet_send.py --descriptors',