Remove EVP_MD_CTX_cleanup and put its functionality into EVP_MD_CTX_init

The idea is that with EVP_MD_CTX_create() and EVP_MD_CTX_destroy(),
EVP_MD_CTX_cleanup and EVP_MD_CTX_init is not used the same as before.
Instead, we need a single function that can be used to reinitialise an
existing EVP_MD_CTX that's been created with EVP_MD_CTX_create()
previously.  Combining EVP_MD_CTX_cleanup and EVP_MD_CTX_init into
that one function is the answer.

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