ecc: Support Montgomery curve for gcry_mpi_ec_mul_point.

* mpi/ec.c (_gcry_mpi_ec_get_affine): Support Montgomery curve.
(montgomery_ladder): New.
(_gcry_mpi_ec_mul_point): Implemention using montgomery_ladder.
(_gcry_mpi_ec_curve_point): Check x-coordinate is valid.
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Given Montgomery curve: b * y^2 == x^3 + a * x^2 + x
CTX->A has (a-2)/4 and CTX->B has b^-1

Note that _gcry_mpi_ec_add_points is not supported for this curve.