Support exposing/tunneling to and from unix sockets

[matrss]

I'd like to expose an existing unix socket on yggstack without having to allocate any local tcp port for it.

Ideally something like this would work:

yggstack ... -remote-tcp 12345:/path/to/unix.socket

and

yggstack ... -local-tcp /path/to/local/unix-socket/to/listen/on:<remote-yggdrasil-ipv6>:12345

such that connecting to /path/to/local/unix-socket/to/listen/on would be transparently tunneled to connect to /path/to/unix.socket.

Since unix sockets support both streaming connections and datagram connections, this should be doable for UDP too.

For the time being, I can sidestep this issue by inserting a socat proxy that connect a local listening port to the unix socket, and vice-versa.