Merge bitcoin/bitcoin#31223: net, init: derive default onion port if a user specified a -port

1dd3af8fbc Add release note for #31223 (Martin Zumsande)
997757dd2b test: add functional test for -port behavior (Martin Zumsande)
0e2b12b92a net, init: derive default onion port if a user specified a -port (Martin Zumsande)

Pull request description:

  This resolves #31133 (setups with multiple local nodes each using a different `-port` no longer working with v28.0, see the issue description for more details) by deriving the default onion listening port to be the value specified by `-port` incremented by 1 (idea by vasild / laanwj).
  Note that with this fix, the chosen `-port` values of two local nodes cannot be adjacent, otherwise there will be port collisions again.

  From the discussion in the linked issue, this was the most popular option, followed by doing nothing and telling affected users to change their setups to use `-bind` instead of `-port`. But more opinions are certainly welcome!

  I think that if we decide to do something about the problem described in the issue, we should do so soon (in 28.1.), so I opened this PR.
  Fixes #31133

ACKs for top commit:
  achow101:
    ACK 1dd3af8fbc
  laanwj:
    Tested ACK 1dd3af8fbc
  tdb3:
    Code review ACK 1dd3af8fbc

Tree-SHA512: 37fda2b23bbedcab5df3a401cf5afce66ae5318fb78f9660f83e3fd075b528e8156d7a0903f9a12ffe97ab5d83860587116b74af28670a1f4c2f0d1be4999f40

test/functional/test_runner.py

@@ -341,6 +341,7 @@ BASE_SCRIPTS = [
     'feature_minchainwork.py',
     'rpc_estimatefee.py',
     'rpc_getblockstats.py',
+    'feature_port.py',
     'feature_bind_port_externalip.py',
     'wallet_create_tx.py --legacy-wallet',
     'wallet_send.py --legacy-wallet',