net: open p2p connections to nodes that listen on non-default ports

By default, for mainnet, the p2p listening port is 8333. Bitcoin Core
has a strong preference for only connecting to nodes that listen on that
port.

Remove that preference because connections over clearnet that involve
port 8333 make it easy to detect, analyze, block or divert Bitcoin p2p
traffic before the connection is even established (at TCP SYN time).

For further justification see the OP of:
https://github.com/bitcoin/bitcoin/pull/23306

doc/README.md

@@ -79,6 +79,7 @@ The Bitcoin repo's [root README](/README.md) contains relevant information on th
 - [Init Scripts (systemd/upstart/openrc)](init.md)
 - [Managing Wallets](managing-wallets.md)
 - [Multisig Tutorial](multisig-tutorial.md)
+- [P2P bad ports definition and list](p2p-bad-ports.md)
 - [PSBT support](psbt.md)
 - [Reduce Memory](reduce-memory.md)
 - [Reduce Traffic](reduce-traffic.md)