[FFE] support cgroup namespace unshare

[Serge Hallyn]

unshare should support a [-c,--cgroup=] option to exploit the new cgroup namespaces which the xenial kernel now supports.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Martin Pitt (pitti)]

FF-wise this looks good to me, approved. But please don't rush this in but forward it upstream first, to avoid the risk of small future changes and thus dragging a CLI incompatibility in an LTS for five years. Karel is usually very responsive. Thanks!

[Serge Hallyn (serge-hallyn)]

PR posted here: https://github.com/karelzak/util-linux/pull/295

My concern is that he may want to wait until cgroup namespaces hit
Linus' kernel. They are currently in linux-next. This may put us
past 16.04 release.

[Martin Pitt (pitti)]

Right, that's fine, but the CLI can still be agreed upon before the PR actually lands. Thanks for forwarding!

[Stéphane Graber (stgraber)]

Upstream argument naming appears to be aligned with the namespace name in /proc, so it's a safe bet that --cgroup will be the final option name.

[Serge Hallyn (serge-hallyn)]

Upstream will merge once cgroup namespaces clear linux-next.

https://github.com/karelzak/util-linux/pull/295#issuecomment-193741900

[Serge Hallyn (serge-hallyn)]

Note this has now been merged upstream.

[Iain Lane (laney)]

It looks like we got this organically, and so can close this bug out?

(Please revert if that's false.)