* cipher/ecc-curves.c (curve_aliases, domain_parms): Add Curve25519.
* tests/curves.c (N_CURVES): It's 22 now.
* src/cipher.h (PUBKEY_FLAG_DJB_TWEAK): New.
* cipher/ecc-common.h (_gcry_ecc_mont_decodepoint): New.
* cipher/ecc-misc.c (_gcry_ecc_mont_decodepoint): New.
* cipher/ecc.c (nist_generate_key): Handle the case of
PUBKEY_FLAG_DJB_TWEAK and Montgomery curve.
(test_ecdh_only_keys, check_secret_key): Likewise.
(ecc_generate): Support Curve25519 which is Montgomery curve with flag
PUBKEY_FLAG_DJB_TWEAK and PUBKEY_FLAG_COMP.
(ecc_encrypt_raw): Get flags from KEYPARMS and handle
PUBKEY_FLAG_DJB_TWEAK and Montgomery curve.
(ecc_decrypt_raw): Likewise.
(compute_keygrip): Handle the case of PUBKEY_FLAG_DJB_TWEAK.
* cipher/pubkey-util.c (_gcry_pk_util_parse_flaglist):
PUBKEY_FLAG_EDDSA implies PUBKEY_FLAG_DJB_TWEAK.
Parse "djb-tweak" for PUBKEY_FLAG_DJB_TWEAK.