Extend CardCaps to contain pw1_max_len and pw3_max_len (and initialize these values from ARD).
Add pinpad_verify(), pinpad_modify(), feature_verify()/feature_modify() to CardClient API.
Expose in card_app (and openpgp-card-sequoia card API).
Adjust opgpcard, opgpcard-pin to ue pinpad reader when available.
The goal of this change is a cleaner structure, and in particular to make it the default for client-code to obtain a CardApp with pre-initialized "capabilities" (that is, init_caps() gets called implicitely).
