sudo: use cache_req for initgroups
    
    This is just blind code change, the next patch will improve it so
    for example we don't do initgroups during query-parsing phase.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/1126
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>