Identify elliptic curves by short name, not NID

Using NIDs is awkward and requires updating pyOpenSSL every time a new
curve is added.  This approach avoids needing to update pyOpenSSL
each time a new curve is added, and it results in more readable code
and a more readable dict ELLIPTIC_CURVE_DESCRIPTIONS.
Stop exposing all the NID and SN objects for elliptic curves, now
that we access them by name.