* agent/cvt-openpgp.c (get_keygrip): Handle Curve25519.
(convert_secret_key, convert_transfer_key): Ditto.
* common/openpgp-oid.c (oidtable): Add Curve25519.
(oid_crv25519, openpgp_oid_is_crv25519): New.
* common/util.h (openpgp_oid_is_crv25519): New.
* g10/ecdh.c (pk_ecdh_encrypt_with_shared_point): Handle the case
with Montgomery curve which uses x-only coordinate.
* g10/keygen.c (gen_ecc): Handle Curve25519.
(ask_curve): Change the API and second arg is to return subkey algo.
(generate_keypair, generate_subkeypair): Follow chage of ask_curve.
* g10/keyid.c (keygrip_from_pk): Handle Curve25519.
* g10/pkglue.c (pk_encrypt): Handle Curve25519.
* g10/pubkey-enc.c (get_it): Handle the case with Montgomery curve.
* scd/app-openpgp.c (ECC_FLAG_DJB_TWEAK): New.
(send_key_attr): Work with general ECC, Ed25519, and Curve25519.
(get_public_key): Likewise.
(ecc_writekey): Handle flag_djb_tweak.