serve sftp: fix checksum detection - Fixes #6351

Before this change, rclone serve sftp operating with a new rclone
after the md5sum/sha1sum detection was reworked to just run a plain
`md5sum`/`sha1sum` command in

3ea82032e7 sftp: support md5/sha1 with rsync.net #3254

Failed to signal to the remote that md5sum/sha1sum wasn't supported as
in

71e172a139 serve/sftp: support empty "md5sum" and "sha1sum" commands

We unconditionally return good hashes even if the remote being served
doesn't support the hash type in question.

This fix checks the hash type is supported and returns an error

    MD5 hash not supported

When the backend is first contacted this will cause the sftp backend
to detect that the hash type isn't available.

Unfortunately this may have cached the wrong state so editing or
remaking the config may be necessary to fix it.

cmd/serve/sftp/connection.go

@@ -101,6 +101,9 @@ func (c *conn) execCommand(ctx context.Context, out io.Writer, command string) (
 		if binary == "sha1sum" {
 			ht = hash.SHA1
 		}
+		if !c.vfs.Fs().Hashes().Contains(ht) {
+			return fmt.Errorf("%v hash not supported", ht)
+		}
 		var hashSum string
 		if args == "" {
 			// empty hash for no input