agent: unknown flags on ssh signing requests cause an error.
* 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.
https://tools.ietf.org/html/draft-miller-ssh-agent-02#section-4.5 says:
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.
- Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
- GnuPG-bug-id: T3880