cache_req: Extend cache_req with wildcard lookups
    
    Related:
        https://fedorahosted.org/sssd/ticket/2553
    
    Adds two new functions to the cache_req API:
        - cache_req_user_by_filter_send
        - cache_req_group_by_filter_send
    
    These functions can be used to retrieve users or groups that match a
    specified filter.
    
    Also renames a variable to avoid constant confusion -- the variable is
    only used for debug output.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>