[PATCH] ecc: support Montgomery curve for gcry_mpi_ec_mul_point

[NIIBE Yutaka]

Subject in the previous mail was wrong (fixed in this mail).

I wanted to say that even though gcry_mpi_ec_add_points won't be
supported, gcry_mpi_ec_mul_point can be supported for Montgomery curve
and it is useful for ECDH.

I don't know if we should support gcry_mpi_ec_curve_point when it's
x-coordinate only for Montgomery curve.  It would be OK always return
true for Montgomery curve.
--