* agent/command-ssh.c (ssh_handler_sign_request): if a flag is passed
during an signature request that we do not know how to apply, return
GPG_ERR_UNKNOWN_OPTION.

If the agent does not support the requested flags, or is otherwise
unable or unwilling to generate the signature (e.g. because it
doesn't have the specified key, or the user refused confirmation of a
constrained key), it must reply with a SSH_AGENT_FAILURE message.