Consolidate elliptic curve cofactor handling

Consolidate elliptic curve cofactor handling

The various checks of the cofactor to be set in EC_GROUP_set_generator()
are a bit all over the place. Move them into a single function and clean
things up a little. Instead of calculating directly with the cofactor
member of the group, use a temporary variable and copy this variable only
if all tests passed. In cryptographic contexts the cofactor almost always
fits if not into a single byte then into a word, so copying is cheap.
Also streamline the computations a bit and remove some binary curve
contortions.

ok jsing