Implement kmem.* cgroup files of memory subsystem

Since 3.8 memory subsystem implements kmem.* (kmem.tcp.* was
introduced since 3.3 though) cgroup files. It limits memory
allocations in the kernel per cgroups.

Note that memory.kmem.slabinfo trows EIO until kmem.limit_in_bytes
is set; we have to take care about that behavior.