scd:openpgp: Allow PKSIGN with keygrip also for OPENPGP.3.

* scd/app-openpgp.c (check_keyidstr): Add optional arg r_use_auth to
test also for OpenPGP.3.
(do_sign): Enable that new mode.

This is very useful to allow gpgsm to use a certificate based on the
OpenPGP card's authentication key. That key is usually used for ssh
but it makes sense to use it also for user certificate based
authentication.

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