scd:openpgp: Send a KEY-TIME status with READKEY

* scd/app-openpgp.c (retrieve_fprtime_from_card): New.
(do_readkey): Send a KEY_TIME status.

It might be easier to fold this into KEYPAIRINFO but for backward
compatibility using a separate status line is safer.

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