Add support for serving unix domain sockets

I would like to be able to use tailcat as a connection mechanism for git-annex' generic p2p transport: https://git-annex.branchable.com/design/generic_p2p_transport/.

This essentially requires three things:

• a stable address -- already available with tailcat genkey
• connecting stdin/stdout of a client with an arbitrary remote service -- already what tailcat <tc-addr> does
• exposing a unix domain socket in a way that multiple clients can establish a connection with it

A workaround would be to first expose git-annex' socket file to a port e.g. via socat and then expose that port via tailcat, but I think it would be valuable to be able to serve socket files directly as they are subject to the unix permission system while ports are open to anyone.

Start by reading the implementation behind tailcat <tc-addr> and tailcat genkey, then trace how connections are accepted and connected to stdin/stdout. Add a way to expose a Unix domain socket that multiple clients can use, while retaining Unix permission checks; done means repeated clients can connect through the socket.