serve sftp: fix crash on unsupported operations (eg Readlink)

Before this change the sftp handler returned a nil error for unknown
operations which meant the server crashed when one was encountered.

In particular the "Readlink" operations was causing problems.

After this change the handler returns ErrSshFxOpUnsupported which
signals to the remote end that we don't support that operation.

See: https://forum.rclone.org/t/rclone-serve-sftp-not-working-in-windows/12209

cmd/serve/sftp/handler.go

@@ -94,7 +94,7 @@ func (v vfsHandler) Filecmd(r *sftp.Request) error {
 		// link.symlink = r.Filepath
 		// v.files[r.Target] = link
 	}
-	return nil
+	return sftp.ErrSshFxOpUnsupported
 }
 
 type listerat []os.FileInfo
@@ -150,5 +150,5 @@ func (v vfsHandler) Filelist(r *sftp.Request) (l sftp.ListerAt, err error) {
 		// }
 		// return listerat([]os.FileInfo{file}), nil
 	}
-	return nil, nil
+	return nil, sftp.ErrSshFxOpUnsupported
 }