Per-container CPU usage in /proc/stat

Per-container CPU usage in /proc/stat

Containers can see utilization of all available CPUs, even if the CPU
is utilized by other containers or by the host. The contents
of `/proc/stat` is shared across the system, except for hiding CPUs
excluded by cpuset. This commit attempts to fix that, but at a cost.
CPU usage is read from cpuacct cgroup, but that accounts only for
`user` and `system` fields from `/proc/stat`. Idle time can be
calculated, but other fields cannot, thus are always set to 0.
Additionally, idle time is based on values from the host, so even
a freshly started container has large initial idle time.

If the cpuacct cgroup is not present, or reading from it fails, LXCFS
will return CPU usage stats from the host as before.

Signed-off-by: Jakub Skokan <jakub.skokan@havefun.cz>