cgroups: more cgroup2 fun

cgroups: more cgroup2 fun

Try too read a valid value from a given cgroup file. If it is a legacy
cgroup hierarchy and we fail to find a valid value we terminate early
and report an error.
The cgroup2 hierarchy however, has different semantics. In a few
controller files it will show the value "max" or simply leave it
completely empty thereby indicating that no limit has been set for this
particular cgroup.  However, that doesn't mean that there's no limit. A
cgroup further up the hierarchy could have a limit set that also applies
to the cgroup we are interested in. So for the unified cgroup hierarchy
we need to keep walking towards the cgroup2 root cgroup and try to parse
a valid value.

Fixes: #389.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>