cgroup: improve support for multiple lxcpaths (v3)
Add a monitor command to get the cgroup for a running container. This
allows container r1 started from /var/lib/lxc and container r1 started
from /home/ubuntu/lxcbase to pick unique cgroup directories (which
will be /sys/fs/cgroup/$subsys/lxc/r1 and .../r1-1), and all the lxc-*
tools to get that path over the monitor at lxcpath.
Rework the cgroup code. Before, if /sys/fs/cgroup/$subsys/lxc/r1
already existed, it would be moved to 'deadXXXXX', and a new r1 created.
Instead, if r1 exists, use r1-1, r1-2, etc.
I ended up removing both the use of cgroup.clone_children and support
for ns cgroup. Presumably we'll want to put support for ns cgroup
back in for older kernels. Instead of guessing whether or not we
have clone_children support, just always explicitly do the only thing
that feature buys us - set cpuset.{cpus,mems} for newly created cgroups.
Note that upstream kernel is working toward strict hierarchical
limit enforcements, which will be good for us.
NOTE - I am changing the lxc_answer struct size. This means that
upgrades to this version while containers are running will result
in lxc_* commands on pre-running containers will fail.
Changelog: (v3)
implement cgroup attach
fix a subtle bug arising when we lxc_get_cgpath() returned
STOPPED rather than -1 (STOPPED is 0, and 0 meant success).
Rename some functions and add detailed comments above most.
Drop all my lxc_attach changes in favor of those by Christian
Seiler (which are mostly the same, but improved).