Process curve name when loading EC keys. Fixes #1336

Not super happy with `CURVE_TYPES` so if anyone has a better suggestion speak up.
We'd have to build a mapping of NID -> curve then by iterating over a list of known curves and calling `OBJ_sn2nid` when the backend is instantiated. Certainly doable, but I prefer the simplicity of the `OBJ_nid2sn` conversion personally. I don't feel strongly about it though so if others agree I can change it.