Add EC_KEY_oct2priv and EC_KEY_priv2oct

New functions EC_KEY_oct2priv and EC_KEY_priv2oct. These are private key
equivalents of EC_POINT_oct2point and EC_POINT_point2oct which convert
between the private key octet format and EC_KEY.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>