Before this change `CNetAddr::ip` was a fixed-size array of 16 bytes, not being able to store larger addresses (e.g. TORv3) and encoded smaller ones as 16-byte IPv6 addresses. Change its type to `prevector`, so that it can hold larger addresses and do not disguise non-IPv6 addresses as IPv6. So the IPv4 address `18.104.22.168` is now encoded as `01020304` instead of `00000000000000000000FFFF01020304`. Rename `CNetAddr::ip` to `CNetAddr::m_addr` because it is not an "IP" or "IP address" (TOR addresses are not IP addresses). In order to preserve backward compatibility with serialization (where e.g. `22.214.171.124` is serialized as `00000000000000000000FFFF01020304`) introduce `CNetAddr` dedicated legacy serialize/unserialize methods. Adjust `CSubNet` accordingly. Still use `CSubNet::netmask` of fixed 16 bytes, but use the first 4 for IPv4 (not the last 4). Only allow subnetting for IPv4 and IPv6.
Completes backport (2/2) of core PR19628:
Depends on D9175.