start: simplify cgroup namespace preservation
start: simplify cgroup namespace preservation

Since we are now dumpable we can open /proc/<child-pid>/ns/cgroup so let's
avoid the overhead of sending around fds.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

ss1h2a3tw/lxc,hallyn/lxc,ss1h2a3tw/lxc,hallyn/lxc,thmo/lxc,hallyn/lxc,thmo/lxc,caglar10ur/lxc,caglar10ur/lxc,thmo/lxc,caglar10ur/lxc,ss1h2a3tw/lxc