[WIP] cgroup: don't special case name=systemd

[alban]

Not tested yet.

Attempt to make the cgroup code work on non-systemd distros such as
Void-Linux.

#1076

/cc @iaguis

[iaguis]

This is Semaphore's /proc/self/cgroup:

Semaphore: ~ $ cat /proc/self/cgroup 
11:name=systemd:/user/1003.user/6.session
10:hugetlb:/user/1003.user/6.session
9:perf_event:/user/1003.user/6.session
8:blkio:/user/1003.user/6.session
7:freezer:/user/1003.user/6.session
6:devices:/user/1003.user/6.session
5:memory:/user/1003.user/6.session
4:cpuacct:/user/1003.user/6.session
3:cpu:/user/1003.user/6.session
2:cpuset:/

So the lowest cgroup controller doesn't have the right path (I'm starting to hate the cpuset controller :D).

Semaphore's Ubuntu installation includes libpam-systemd which sets up the name=systemd controller but it does it when the system is already booted so it has the highest hierarchy_id.

I suggest we try to find name=systemd first and fall back to your solution if we can't.

[jonboulle]

ping on this one?

[iaguis]

Right now systemd-nspawn requires a name=systemd cgroup controller mounted on /sys/fs/cgroup/systemd anyway, so even if we implement this it won't solve the problem.

Closing.