scd:p15: Cache the PIN.

* scd/app-p15.c (struct prkdf_object_s): Add flag pin_verified.
(verify_pin): Make use of it.

Theee is still a problem with the APDUs we send: Switching between
signing and decryption does work but not in the other way.

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