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:

backend/webdav/webdav.go

@@ -159,7 +159,9 @@ Set to 0 to disable chunked uploading.
 			Help:     "Exclude ownCloud mounted storages",
 			Advanced: true,
 			Default:  false,
-		}},
+		},
+			fshttp.UnixSocketConfig,
+		},
 	})
 }
 
@@ -177,6 +179,7 @@ type Options struct {
 	ChunkSize          fs.SizeSuffix        `config:"nextcloud_chunk_size"`
 	ExcludeShares      bool                 `config:"owncloud_exclude_shares"`
 	ExcludeMounts      bool                 `config:"owncloud_exclude_mounts"`
+	UnixSocket         string               `config:"unix_socket"`
 }
 
 // Fs represents a remote webdav
@@ -458,7 +461,12 @@ func NewFs(ctx context.Context, name, root string, m configmap.Mapper) (fs.Fs, e
 		precision:   fs.ModTimeNotSupported,
 	}
 
-	client := fshttp.NewClient(ctx)
+	var client *http.Client
+	if opt.UnixSocket == "" {
+		client = fshttp.NewClient(ctx)
+	} else {
+		client = fshttp.NewClientWithUnixSocket(ctx, opt.UnixSocket)
+	}
 	if opt.Vendor == "sharepoint-ntlm" {
 		// Disable transparent HTTP/2 support as per https://golang.org/pkg/net/http/ ,
 		// otherwise any connection to IIS 10.0 fails with 'stream error: stream ID 39; HTTP_1_1_REQUIRED'