cgroups: don't mount under init's cgroup

1. deeper hierarchy has steep performance costs
2. init may be under /init, but containers should be under /lxc
3. in a nested container we like to bind-mount $cgroup_path/$c/$c.real
   into $cgroup_path - but task 1's cgroup is $c/$c.real, so a nested
   container would be in $c/$c.real/lxc, which would become
   /$c/$c.real/$c/$c.real/lxc when expanded
4. this pulls quite a bit of code (of mine) which is always nice

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>