Enhancement: Support connection to rest-server using unix socket

Restic now supports using a unix socket to connect to a rest-server
version 0.13.0 or later. This allows running restic as follows:

```
rest-server --listen unix:/tmp/rest.socket --data /path/to/data &
restic -r rest:http+unix:///tmp/rest.socket:/my_backup_repo/ [...]
```

https://github.com/restic/restic/issues/4287
https://github.com/restic/restic/pull/4655
