agent: Fix YK s/n and prettify the request card prompt for Yubikeys
* agent/divert-scd.c (ask_for_card): Detect and re-format the Yubikey prompt. * scd/app.c (app_munge_serialno): Fix Yubikey s/n munging. (card_get_dispserialno): Ditto. * scd/app-openpgp.c (get_disp_serialno): Remove. (get_prompt_info): Use app_get_dispserialno.--