Implement setns for cgroupns by path

[alban]

The OCI runtime spec allows to specify a cgroupns path to enter:
https://github.com/opencontainers/runtime-spec/blob/master/config-linux.md#namespaces

Example:

    "namespaces": [
        {
            "type": "cgroup",
            "path": "/proc/1234/ns/cgroup"
        },

Signed-off-by: Alban Crequy alban@kinvolk.io

---

Issue found while trying to implement a test for this:
opencontainers/runtime-tools#572 (comment)

[cyphar]

Thanks for this, but we're already working on it as part of #1184 (which I'm working on carrying at the moment). In short (you can read the discussion in the thread), the "simple" way of implementing cgroup namespaces doesn't work as you'd expect because you will end up with having the root of the cgroup namespace being the cgroup the process was in before runc put it in a separate cgroup.

I'm closing this as a dup of #1184 (and I am going to carry it in the near future).