EVP_FETCH: deal with names without pre-defined NIDs

We didn't deal very well with names that didn't have pre-defined NIDs,
as the NID zero travelled through the full process and resulted in an
inaccessible method.  By consequence, we need to refactor the method
construction callbacks to rely more on algorithm names.

We must, however, still store the legacy NID with the method, for the
sake of other code that depend on it (for example, CMS).

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8878)