cache_req: introduce cache_behavior enumeration
    
    Instead of using individual booleans for controlling the behavior
    of the nss reponder with regard to cache usage, use a single
    enumeration that can be extended to add new behaviors as needed.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/4098
    
    Signed-off-by: Simo Sorce <simo@redhat.com>