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.
_backend is not a property on a backend
mark named curve inside EC key classes
british english? never heard of it.
Merge pull request #1538 from reaperhulk/named-curve

Set OPENSSL_EC_NAMED_CURVE on our EC_KEY instances