cgroup: refuse to return accounting data if accounting isn't turned on

cgroup: refuse to return accounting data if accounting isn't turned on

We used to be a bit sloppy on this, and handed out accounting data even
for units where accounting wasn't explicitly enabled. Let's be stricter
here, so that we know the accounting data is actually fully valid. This
is necessary, as the accounting data is no longer stored exclusively in
cgroupfs, but is partly maintained external of that, and flushed during
unit starts. We should hence only expose accounting data we really know
is fully current.