KDF: use string names not macros to align with other algorithms.

KDF: use string names not macros to align with other algorithms.

Only the KDF and PRF algorithms used the macros for their names, all other
algorithms used a string name directly.  This brings the KDFs and PRFs into
line with the rest.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10293)