agent: Add A-flag for KEYINFO output for card.

* agent/command.c (do_one_keyinfo): Add ON_CARD argument to put
A-flag.
(cmd_keyinfo): Call agent_card_keyinfo to offer additional information
if it's on card.

This is a modification in gpg-agent, intended for better
enum_secret_keys in gpg frontend.

• GnuPG-bug-id: T4244
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>