Enhancement: Support listening on a unix socket

It is now possible to make rest-server listen on a unix socket by prefixing
the socket filename with `unix:` and passing it to the `--listen` option,
for example `--listen unix:/tmp/foo`.

This is useful in combination with remote port forwarding to enable a remote
server to backup locally, e.g.:

```
rest-server --listen unix:/tmp/foo &
ssh -R /tmp/foo:/tmp/foo user@host restic -r rest:http+unix:///tmp/foo:/repo backup
```

https://github.com/restic/rest-server/pull/272