* scd/app-common.h (yubikey_get_serialno): New.
* scd/app-openpgp.c (yubikey_get_serialno): New.
* scd/app.c (card_get_serialno): Use OpenPGP app's serialno,
when it's enabled for Yubikey.
(send_serialno_and_app_status): Use card_get_serialno, not
directly accessing ->serialno.