Provide key_fetch routine that will not search subkey fingerprints

Provide key_fetch routine that will not search subkey fingerprints

We have *_key_fetch_fp to search for keys by fingerprint, but that
includes subkeys. While unlikely a subkey could be bound to multiple
certification primary keys, resulting in multiple keys being returned
for a fingerprint query. Provide a plain *_fetch_key that only searches
primary keys and is thus guaranteed to return at most one key.