Adds low-level support for launching Linux containers with cgroup namespaces.
* gnu/build/linux-container.scm (%namespaces): Add 'cgroup.
(namespaces->bit-mask): Handle it.
* guix/build/syscalls.scm (CLONE_NEWCGROUP): New variable.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>