agent: Support SSH signature flags.
* agent/command-ssh.c (SSH_AGENT_RSA_SHA2_256): New. (SSH_AGENT_RSA_SHA2_512): New. (ssh_handler_sign_request): Override SPEC when FLAGS is specified.
- GnuPG-bug-id: T3880
- Reported-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>