Make unified hierarchy mountable on 4.5.0+ kernels
Make unified hierarchy mountable on 4.5.0+ kernels

Starting from kernel 4.5.0 the unified hierarchy is accessible via a
separate "cgroup2" file system type.
The same kernel version also removed the ability to mount it via a
"__DEVEL__sane_behavior" option provided to the "cgroup" file system type.

Adapt to these changes by first trying to mount the "cgroup2" file system,
then, if this file system type is not known, by trying to mount the unified
hierarchy the old way via the "__DEVEL__sane_behavior" mount option.

Signed-off-by: Maciej S. Szmigiero <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@maciej.szmigiero.name>

lxc/cgmanager,lxc/cgmanager