stage1: support hybrid cgroup hierarchy

[iaguis]

systemd introduced the hybrid cgroup hierarchy that mounts a cgroup v1
and v2 hierarchy in parallel. After systemd v233 the cgroup v2 hierarchy
is mounted in /sys/fs/cgroup/unified. This breaks rkt (specifically
flavor host) because we mount /sys/fs/cgroup ourselves and were not
creating the unified directory.

Let's create /sys/fs/cgroup/unified in CreateCgroup() to support the
hybrid cgroup hierarchy. If the host system (or the stage1) doesn't use
the hybrid hierarchy, having this directory created doesn't hurt.

Fixes #3741

[iaguis]

I tried other implementations before deciding to submit this one, you can read them in git history form here: https://github.com/kinvolk/rkt/commits/iaguis/hybrid-hierarchy-story

[alban]

@iaguis It looks good but...

CreateCgroups() is called twice:

• one time with root = "/" to support ancient distributions
• one time on the future root of the pod.

This double usage already non-trivial error management when mounting sysfs.

And this new mkdir is only required on the future root of the pod and not on the root of the host. I wonder if we can reorganise the code to avoid the mkdir on the host.

[iaguis]

I thought about that and we could create the dir only if if root != "/" so we don't pollute the host fs.

However, note that we're creating the directory in atmpfs inside a mount ns that's different from the host's so I decided it's not worth it.

[alban]

LGTM if green