scd:openpgp: Use a special compare for the serialno.

* scd/app-openpgp.c (check_keyidstr): Ignore the card version and also
compare case insensitive.
(do_learn_status): Add mssing error handling.

This is required because we changed what we emit as serialno of
OpenPGP cards but existing keys still use the old form of the serial
number (i.e. with a firmware version). This is so that existing stub
keys of gpg-agent will continue to work.

• GnuPG-bug-id: T5100