Support unix socket addresses, and possibly arbitrary other address types (?)

[armanbilge]

This is almost definitely a breaking change, so just something to keep in mind for the future.

Seems like there are a plethora of socket types, which have different kinds of addressing. e.g. AF_UNIX, AF_INET, AF_INET6. Don't know if any of the others are important.

https://man7.org/linux/man-pages/man2/socket.2.html

Currently in FS2 we have an awkward SocketGroup and UnixSockets, when (in theory) they can actually be sharing the same machinery. Also, it's possible to use UDP with unix sockets, which the current FS2 encoding also does not support.