EC: Refactor ec_curve_name2nid() to accept NIST curve names

EC: Refactor ec_curve_name2nid() to accept NIST curve names

We can find no reason why everyone should have to call both
EC_curve_nist2nid() and ec_curve_name2nid() to find the NID for a
name, and it's too easy to forget EC_curve_nist2nid(), so we make life
simpler.

One could argue that FIPS only allows a limited set of names, but that
now gets handled internally, and those who really want to be really
sure to only get the NIST names can still do so with EC_curve_nist2nid()

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11391)