Cgroups.Resources.Unified support wrt systemd driver

[kolyshkin]

Initial support for cgroupv2 unified resources was merged in #2584.

With that in place, one can set, say, linux.resources.unified.pids.max = 42 which will make runc to write 42 to pids.max. The problem is, systemd TasksMax will still be unset/unlimited.

The issue can be solved by adding a map of some known "unified" parameters
to systemd properties.

Alternatively, we can forget about systemd properties and implement #2602 instead (which AFAIK is what crun does).

[AkihiroSuda]

implement #2602 instead (which AFAIK is what crun does).

👍

[kolyshkin]

Copy/pasting my comment from #2602 (comment)

OK so we have discussed it with @mrunalp and @giuseppe and looks like we can proceed with the plan outlined in #2603.

The biggest concern about switching systemd drivers to create a sub-cgroup is that software that uses libcontainer (kubelet etc.) might break. We could do it as an option but I don't think there will be any benefit.

So that's the plan for now