scd: KEYNO cleanup.
* scd/app-openpgp.c (get_public_key, send_keypair_info, do_readkey) (change_keyattr, change_keyattr_from_string, ecc_writekey, do_genkey) (compare_fingerprint, check_against_given_fingerprint): KEYNO starts from 0.
Backport of master commit: fd689e85423d0d80d725f0315c52d94f0e9766f8