agent: Cleanups to prepare implementation of Ed25519.
* agent/cvt-openpgp.c: Remove. (convert_to_openpgp): Use gcry_sexp_extract_param. * agent/findkey.c (is_eddsa): New. (agent_is_dsa_key, agent_is_eddsa_key): Check whether ecc means EdDSA. * agent/pksign.c (agent_pksign_do): Add args OVERRIDEDATA and OVERRIDEDATALEN. * common/ssh-utils.c (is_eddsa): New. (get_fingerprint): Take care or EdDSA.