Rename the Elliptic Curves extension to supported_groups

This is a skin deep change, which simply renames most places where we talk
about curves in a TLS context to groups. This is because TLS1.3 has renamed
the extension, and it can now include DH groups too. We still only support
curves, but this rename should pave the way for a future extension for DH
groups.

Reviewed-by: Rich Salz <rsalz@openssl.org>