nspawn: when in hybrid mode, chown() both the legacy and the unified hierarchy...

nspawn: when in hybrid mode, chown() both the legacy and the unified hierarchy to the root in the container

If user namespacing is used, let's make sure that the root user in the
container gets access to both /sys/fs/cgroup/systemd and
/sys/fs/cgroup/unified.

This matches similar logic in cg_set_access().