Internal types only: enum -> macros.
Nettle.ECC_Curve: Improved documentation about point_mul().

Added note that point_mul() validates that the coordinate
is valid for the curve.
SSL.Cipher [ecdh]: Added note about point validation.

There are attacks against ECDH/ECDHE that are based on providing
points that aren't on the negotiated curve, so add an explicit
note that the received points are validated in point_mul().