Set OPENSSL_EC_NAMED_CURVE on our EC_KEY instances

This means any X.509 certs generated from our keys will be encoded
along with the curve OID so that we can still load them afterwards.

http://wiki.openssl.org/index.php/Elliptic_Curve_Cryptography#Named_Curves
Can we centralize this into a single `def _mark_named_ec_curve` method, and then add a comment explaining what this does there?