Feature: Support unix sockets

[ChrisLundquist]

An example unix socket server:
https://gist.github.com/teknoraver/5ffacb8757330715bcbcc90e6d46ac74

The punch line is:

	server := http.Server{
		Handler: http.FileServer(http.Dir(root)),
	}

	unixListener, err := net.Listen("unix", os.Args[1])
	if err != nil {
		panic(err)
	}
	server.Serve(unixListener)

I tried this here and sadly our types don't match net.http's interface.
From: https://golang.org/pkg/net/http/#Server
func (srv *Server) Serve(l net.Listener) error

However here, we're asking for func Serve(servers ...*http.Server)
and then we hardcode tcp

		l, err := a.net.Listen("tcp", s.Addr)

here in listen

It seems like we should match net.http here and take a https://golang.org/pkg/net/#Listener which would allow us to grab https://golang.org/pkg/net/#Addr so we can open tcp/udp/unix as appropriate.

I could be missing something, but I didn't see anything too TCP specific in the fork/exec/inherit code.

My use case is that I want to front multiple versions of the same service with nginx, and I don't want to play games with ports.
While that particular case is admittedly a little wacky, fronting a service with nginx or the like isn't too unreasonable (say for handling auth somewhere else).