webdav: add --webdav-unix-socket-path to connect to a unix socket

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:

docs/content/webdav.md

@@ -294,6 +294,17 @@ Properties:
 - Type:        bool
 - Default:     false
 
+#### --webdav-unix-socket
+
+Path to a unix domain socket to dial to, instead of opening a TCP connection directly
+
+Properties:
+
+- Config:      unix_socket
+- Env Var:     RCLONE_WEBDAV_UNIX_SOCKET
+- Type:        string
+- Required:    false
+
 #### --webdav-description
 
 Description of the remote.