Replumbing: pre-populate the EVP namemap with commonly known names

Replumbing: pre-populate the EVP namemap with commonly known names

This adds ossl_namemap_empty(), to detect if a namemap is empty and
can thereby be pre-populated.

This also affects the way legacy NIDs are looked up in
evp_cipher_from_dispatch() and evp_md_from_dispatch().  Instead of
trying to find the NID directly, look up the legacy method structure
and grab the NID from there.  The reason is that NIDs can be aliases
for other NIDs, which looks like a clash even if wasn't really one.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8984)