scd: Allow standard keyref scheme for app-openpgp.

* scd/app-openpgp.c (do_change_pin): Allow prefixing the CHVNO with
"OPENPGP."

The generic keyref allows for better error detection in case a keyref
is send to a wrong card. This has been taken from master commit
3231ecdafd71ac47b734469b07170756979ede72 which has additional changed
for gpg-card-tool, which is only available there.

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