scd: New getinfo sub-command apdu_strerror.

* scd/apdu.c (apdu_strerror): Add missing status codes.
* scd/command.c (cmd_getinfo): New sub-command apdu_strerror.

This is quite handy for gpg-card's APDU command and avoids that we
need to duplicate the mapping table or put it into a shared file.