gpg: New card function agent_scd_keypairinfo.

* g10/call-agent.c (scd_keypairinfo_status_cb)
(agent_scd_keypairinfo): New.  Taken from gpgsm.

(cherry picked from commit 0fad61de159acf39e38a04f28f162f0beb0e77d6)

• Signed-off-by: Werner Koch <wk@gnupg.org>