Enhancement: can now listen on a unix socket

If `--listen unix:/tmp/foo` is passed, the server will listen on a unix socket. This is triggered by the prefix `unix:`.

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

```bash
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