scd:p15: Support special extended usage flags for OpenPGP keys.

* scd/app-p15.c (struct gpgusage_flags_s): New.
(struct prkdf_object_s): Add field gpgusage.
(struct app_local_s): Add field any_gpgusage.
(dump_gpgusage_flags): New.
(read_p15_info): Parse athe gpgusage flags.
(do_getattr): Take care of the gpgusage flags.

This features allows to mark keys to be used for OpenPGP. This is
done by putting additional extended key infos into the certificate
stored on the cards. Only if any such extended key usage is detected
for a stored certificate this new mode chimes in.

This feature gives the card issuer a high flexibility on how to make
sure certain keys are used with OpenPGP.