This adds a new optional parameter to the backend, to specify a path
to a unix domain socket to connect to, instead the specified URL.
The URL itself is still used for the rest of the HTTP client, allowing
host and subpath to stay intact.
This allows using rclone with the webdav backend to connect to a WebDAV
server provided at a Unix Domain socket:
rclone serve webdav --addr unix:///tmp/my.socket remote:path
rclone --webdav-unix-socket /tmp/my.socket --webdav-url http://localhost lsf :webdav:
3ffa47ea16