forbid writes and reads to the root cgroup or cuttr's cgroups
Change in the check_path interface: there are now two different methods, check_file and check_dir. check_dir is the previous check_path. check_file checks that the file is not inside a root cgroup and it also checks that it is not in one of cuttr's cgroups. This is to make sure that a user cannot make any changes to the values which are not his.