> net: change assumed I2P port to 0
>
> * When accepting an I2P connection, assume the peer has port 0 instead
> of the default 8333 (for mainnet). It is not being sent to us, so we
> must assume something.
> * When deriving our own I2P listen CService use port 0 instead of the
> default 8333 (for mainnet). So that we later advertise it to peers
> with port 0.
>
> In the I2P protocol SAM 3.1 and older (we use 3.1) ports are not used,
> so they are irrelevant. However in SAM 3.2 and newer ports are used and
> from the point of view of SAM 3.2, a peer using SAM 3.1 seems to have
> specified port=0.
> net: distinguish default port per network
>
> Change `CChainParams::GetDefaultPort()` to return 0 if the network is
> I2P.
> net: do not connect to I2P hosts on port!=0
>
> When connecting to an I2P host we don't specify destination port and it
> is being forced to 0 by the SAM 3.1 proxy, so if we connect to the same
> host on two different ports, that would be actually two connections to
> the same service (listening on port 0).
>
> Fixes https://github.com/bitcoin/bitcoin/issues/21389
> test: ensure I2P ports are handled as expected
> doc: mention that we enforce port=0 in I2P
>
> Co-authored-by: Jon Atack <jon@atack.com>
This is a backport of core#22112 and core#22497 (revert one commit of core22112)