Introduce ec_decode_scalar()

Introduce ec_decode_scalar()

This is a helper that decodes a scalar from field-internal representation
to a representation as a BIGNUM in the interval [0, p). This simplifies
EC_GROUP_get_curve() and EC_POINT_get_Jprojective_coordinates() to a few
obvious lines and prepares cleanup in EC_POINT_get_affine_coordinates().

ok jsing