gpg: Use "SCD KEYINFO" to get available card keys.
* g10/skclist.c (enum_secret_keys): Don't use agent_scd_cardlist and agent_scd_serialno, but agent_scd_keyinfo.
When there are multiple cards/tokens, this change can avoid switching
card of foreground access.
- GnuPG-bug-id: T4784
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>