scd:p15: Allow to use an auth object label with cmd CHECKPIN.

* scd/app-p15.c (prepare_verify_pin): Allow for PRKDF to be NULL.
(make_pin_prompt): Ditto.
(verify_pin): Ditto.
(do_check_pin): Allow using the Label to specify a PIN.

Note that a label may not work in all cases because we can't select
the protected object's EF first.

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