support unix-sockets as hidden-service endpoints
It would be nice if hidden-service setup supported proxying to a local unix-socket. nginx, Twisted (and probably others) support this for HTTP for example.
This would reduce the chances of "accidentally" listening on a public TCP port (and eases the task of verifying this) and is somewhat faster.
I'm unsure if it's better to have a new config-item, or overlap with the existing hidden-service listening specification.