Prepare to provide EC_GROUP_{get,set}_curve(3)

Prepare to provide EC_GROUP_{get,set}_curve(3)

There are numerous functions in ec/ that exist with _GF2m and _GFp
variants for no good reason.  The code of both variants is the same.
The EC_METHODs contain a pointer to the appropriate version.  This
commit hides the _GF2m and _GFp variants from internal use and
provides versions that work for both curve types. These will be made
public in an upcoming library bump.

Similar to part of OpenSSL commit 8e3cced75fb5fee5da59ebef9605d403a999391b

ok jsing