mount: change interface of mount commands to take mount options

This is in preparation of being able to pass mount options to the rc
command "mount/mount"

backend/cache/cache_mount_windows_test.go

@@ -41,7 +41,7 @@ func (r *run) mountFs(t *testing.T, f fs.Fs) {
 	options := []string{
 		"-o", "fsname=" + device,
 		"-o", "subtype=rclone",
-		"-o", fmt.Sprintf("max_readahead=%d", mountlib.MaxReadAhead),
+		"-o", fmt.Sprintf("max_readahead=%d", mountlib.Opt.MaxReadAhead),
 		"-o", "uid=-1",
 		"-o", "gid=-1",
 		"-o", "allow_other",