ecc: Support the non-standard 0x40 compression flag for EdDSA.
* cipher/ecc.c (ecc_generate): Check the "comp" flag for EdDSA. * cipher/ecc-eddsa.c (eddsa_encode_x_y): Add arg WITH_PREFIX. (_gcry_ecc_eddsa_encodepoint): Ditto. (_gcry_ecc_eddsa_ensure_compact): Handle the 0x40 compression prefix. (_gcry_ecc_eddsa_decodepoint): Ditto. * tests/keygrip.c: Check an compresssed with prefix Ed25519 key. * tests/t-ed25519.inp: Ditto.