Use systemd for cgroup2 split-tunneling

[hulthe]

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This change is 

[dkwo]

Would this change imply that mullvad-exclude will only work with systemd?
is there any other solution that would not require that?
cc: @meator @MarkusPettersson98

[hulthe]

@dkwo This change requires systemd, yes.

We will have a fallback to the old net_cls cgroup. This would apply to distros with old kernels, or without systemd. This fallback might be removed in the future though, since we don't really want to maintain support for deprecated kernel interfaces.

We're thinking about adding a fallback to some other non-systemd mechanism, but no decision yet.

[dkwo]

I see. Just keep in mind that the current net_cls cgroup method is already broken e.g. on runit, so I would not bother keeping it around.

[dkwo]

Another clarification: is the requirement for the init/service manager to implement cgroups v2, or specifically to have systemd as pid1? in the former case, runit also has support for cgroups v2, and others too.