In legacy cgroups mode, we were running `mountinfo -q ""` which was
generating an error message. If we return immediately when
cgroup2_find_path returns an empty value, we avoid this message.
prior to cgroups getting mounted, /sys/fs/cgroup will still exist,
but attempts to make directories in it will fail, change cgroup2_set_limits() to
verify that cgroups are mounted instead of just checking that /sys/fs/cgroup
exists.
This fixes#307.
This fixes#321.
