ecc: Fix X25519 computation on Curve25519.
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ecc: Fix X25519 computation on Curve25519.

* cipher/ecc.c (ecc_encrypt_raw): Tweak of bits when
PUBKEY_FLAG_DJB_TWEAK is enabled.
(ecc_decrypt_raw): Return 0 when PUBKEY_FLAG_DJB_TWEAK is enabled.
* tests/t-cv25519.c (test_cv): Update by using gcry_pk_encrypt.

X25519 function is not a plain scalar multiplication, but does
two things; the scalar bits are tweaked before applying scalar
multiplication and X0 function is applied to the result of
scalar multiplication.

In libgcrypt, _gcry_mpi_ec_mul_point is a plain scalar multiplication
and those two things are done in functions for ECDH with X25519.