Use systemd API to create cgroup

[PhilippWendler]

Currently we create cgroups ourselves, but the kernel is moving in the direction of allowing only a single process to manipulate the cgroup hierarchy. This would be systemd in practice. So we should start migrating to use systemd's dbus API for creating cgroups, and later on for all cgroup accesses.

Documentation on this is available here, we should be able to use a "scope". We can also look at systemd-run to see how this can be done.

Because scopes can be created by ordinary users, this even has the advantage that in the future we might be able to install and use BenchExec on systems as regular users without any special permissions, which would be great.

[danieldietsch]

Would this mean that benchexec then depends on systemd? If so, what about Linux systems without systemd?

[PhilippWendler]

No, of course we can simply keep the existing code for systems without systemd.

[danieldietsch]

Sounds good, thanks!

[PhilippWendler]

Part of #791 for cgroupsv2. Will not be implemented for cgroupsv1.