Added support for connecting to a Unix domain socket.

This PR adds a connect_unix function to support Unix domain sockets.

I'm using it build Docker bindings, which runs over a Unix socket by default:

https://github.com/arjunguha/ocaml-docker

If I can get this merged in some form, I'd want to open a PR for cohttp next:

https://github.com/arjunguha/ocaml-cohttp/commit/d918afa7c682a6c4a13d4030fe07aa2b216c3230