scd: Detect Yubikey and provide nicer display-s/n.
* scd/app-common.h (struct app_ctx_s): Rename unused field card_version to cardversion. * scd/app.c (app_new_register): Add code rom 2.3 to detect the Yubikey and set cardversion. (app_get_dispserialno): New. * scd/app-openpgp.c (do_getattr): Use app_get_dispserialno.