cgroup: fine tune when to apply cgroup attributes to the root cgroup

cgroup: fine tune when to apply cgroup attributes to the root cgroup

Let's tweak when precisely to apply cgroup attributes on the root
cgroup.

With this we now follow the following rules:

1. On cgroupsv2 we never apply any regular cgroups to the host root,
   since the attributes generally do not exist there.

2. On cgroupsv1 we do not apply any "weight" or "shares" style
   attributes to the host root cgroup, since they don't make much sense
   on the top level where there's only one group, hence no need to
   compare weights against each other. The other attributes are applied
   to the host root cgroup however.

3. In any case we don't apply attributes to the root of container
   environments (and --user roots), under the assumption that this is
   managed by the manager further up. (Note that on cgroupsv2 this is
   even enforced by the kernel)

4. BPF pseudo-attributes are applied in all cases (since we can have as
   many of them as we want)