agent: Support --mode=ssh option for CLEAR_PASSPHRASE.
* agent/command.c (cmd_clear_passphrase): Add support for SSH.
(cherry picked from commit ae966bbe9b16ed68a51391afdde615339755e22d)
- GnuPG-bug-id: T4340
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>