Refactor TLS1-PRF to create the MAC contexts early

Refactor TLS1-PRF to create the MAC contexts early

The TLS1-PRF implementation fetched the digest(s) for the underlying
MAC, just to get their names and pass those down to the MAC, which in
turn would fetch those same digests again.

This change circumvents this by fetching the MAC (or MACs in the
MD5-SHA1 special case) and create the MAC contexts for them directly
when this PRF receives the relevant parameters, thus only having to
pass EVP_MAC_CTX pointers around.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9930)