CACHE_REQ: Export cache_req_search_ncache_add() as cache_req private interface
    
    Previously, it was enough to add an entry to the negative cache of the
    domain being processed in cache_req (cr->domain). But the locator plugin
    can return any domain from the processed domain's subdomain list as
    well.
    
    Therefore, this patch extends the internal API for the possibility of
    setting the negative cache in another domain as well.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>