Add linux.resources.devices

[wking]

This cherry-picks the device-cgroup commit from #98, since I think the
approach is useful regardless of how we're creating the devices.

[wking]

And all the pre-close pushback against #98 seemed to focus on the commit dropping linux.devices. I didn't see any comments in #98 on the commit cherry-picked here (just the discussion in #94: 1, 2, 3, 4, 5).

[wking]

I was hunting around for the device docs to link today, and was surprised to find them in the runtime.json tree. Besides avoiding the complication of overloading (see #101), I think splitting the mknod and cgroup functionality would let us move the mknod functionality back into the config.json tree (it certainly doesn't seem like a host-specific setting), while leaving the cgroup functionality in runtime.json (since that does seem like a host-specific setting).

[crosbymichael]

I think this one can be closed now that we already have a Device struct and fields documented.

[wking]

On Fri, Sep 25, 2015 at 01:45:09PM -0700, Michael Crosby wrote:

I think this one can be closed now that we already have a Device
struct and fields documented.

Not really documented, see #101 ;). I still think the separation
between mknod and cgroups proposed here would make everyone's life
easier than landing the #101 docs for the current structure. But it's
your project, I'm just floating suggestions :).