pnNetCommon is also the only library that actually links with Winsock,
so including the header elsewhere doesn't make much sense.

Although AF_INET is always 2 on every relevant system, it's better not
to assume this. Here, we use AF_INET only as a marker value in a
serialization format and never pass it to any system functions, so it's
more important that the value is consistent between systems than that it
matches the system constant.

Only has kInet (AF_INET, IPv4) for now. This will be relevant if IPv6
support is ever added here - unlike AF_INET, which is 2 on all relevant
OSes, AF_INET6 has a different value on Windows, macOS, and Linux.