cache_req: preparations for different input type
    
    Currently cache_req takes only user name as an input parameter. However,
    this is not enough since we will need also UID and GID in the future.
    This patch creates a structure to hold input parameters so it can be
    simply extended to support other input types.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>