Revert "kdf: make function naming consistent."

Revert "kdf: make function naming consistent."

The commit claimed to make things more consistent. In fact it makes it
less so. Revert back to the previous namig convention.

This reverts commit 765d04c9460a304c8119f57941341a149498b9db.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12186)