nspawn: introduce the new /machine/ tree in the cgroup tree and move containers there
Containers will now carry a label (normally derived from the root directory name, but configurable by the user), and the container's root cgroup is /machine/<label>. This label is called "machine name", and can cover both containers and VMs (as soon as libvirt also makes use of /machine/). libsystemd-login can be used to query the machine name from a process. This patch also includes numerous clean-ups for the cgroup code.
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