cgroup: check whether unified hierarchy is writable

cgroup: check whether unified hierarchy is writable

When systemd is running inside a container employing user
namespaces it currently mounts the unified cgroup hierarchy
without being able to write to it. This causes systemd to
freeze during boot.
This patch checks whether the unified cgroup hierarchy
is writable. If it is not it will not mount it.

This solution is based on a patch by Evgeny Vereshchagin.

Closes #6408.
Closes https://github.com/lxc/lxc/issues/1678 .