ecc: Use opaque MPI for 'd' of Ed25519/EdDSA.
* cipher/ecc-curves.c (mpi_ec_setup_elliptic_curve): Add FLAGS. Use opaque MPI for Ed25519/EdDSA, too. (_gcry_mpi_ec_internal_new): Follow the change. (_gcry_mpi_ec_new): Likewise.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>