scd:p15: Support decryption with CardOS 5 cards.

* scd/app-p15.c (do_decipher): New.

tested using the D-TRUSt card and a SCR3310 reader. The Kobil KAAN
Advanced, I used for the signing tests could not be used because it
supports only Short APDU Level exchange.

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