scd: Add handling of Ed448 key.

* scd/app-openpgp.c (struct app_local_s): Add ecc.algo field.
(send_key_attr): Use ecc.algo field.
(ecc_read_pubkey): Use ecc.algo field.
(ecc_writekey): Ed448 means EdDSA.
(parse_algorithm_attribute): Set ecc.algo field from card.
Add checking for Ed25519 for ECC_FLAG_DJB_TWEAK flag.

There used to be a possible support of Ed25519 with ECDSA, (instead of
EdDSA). To distinguish key for Ed25519 for EdDSA, we use the
flag: (flags eddsa). Ed448 has no support for ECDSA and defaults to
EdDSA even if no such flag.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>