agent: Add --card option for READKEY.
* agent/findkey.c (agent_write_shadow_key): New. * agent/command-ssh.c (card_key_available): Use agent_write_shadow_key. * agent/learncard.c (agent_handle_learn): Likewise. * agent/command.c (cmd_readkey): Add --card option.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>