serve sftp: return not supported error for not supported commands

Before this change, if a hardlink command was issued, rclone would
just ignore it and not return an error.

This changes any unknown operations (including hardlink) to return an
unsupported error.

cmd/serve/sftp/handler.go

@@ -83,6 +83,10 @@ func (v vfsHandler) Filecmd(r *sftp.Request) error {
 		// link.symlink = r.Filepath
 		// v.files[r.Target] = link
 		return sftp.ErrSshFxOpUnsupported
+	case "Link":
+		return sftp.ErrSshFxOpUnsupported
+	default:
+		return sftp.ErrSshFxOpUnsupported
 	}
 	return nil
 }