scd:openpgp: Add attribute "UIF" for convenience.

* scd/app-openpgp.c (do_getattr): New attrubute "UIF".
(do_learn_status): Use that.

Actually this is not just convenience but will make it easier to add
new keys to an openpgp card - we will need to change this only at one
place.

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