New function EVP_MD_free()

New function EVP_MD_free()

This function re-implements EVP_MD_meth_free(), but has a name that
isn't encumbered by legacy EVP_MD construction functionality.

We also refactor most of EVP_MD_meth_new() into an internal
evp_md_new() that's used when creating fetched methods.

EVP_MD_meth_new() and EVP_MD_meth_free() are rewritten in terms of
evp_md_new() and EVP_MD_free().  This means that at any time, we can
deprecate all the EVP_MD_meth_ functions with no harmful consequence.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9758)