cgroups: get controllers from /proc/self/cgroups (v2)

Some controllers can be disabled in kernel options. In this case they
are shown in /proc/cgroups, but they could not be mounted.

All enabled controllers can be collected from /proc/self/cgroup.

https://github.com/xemul/criu/issues/28



v2: ',' is used to separate controllers

Cc: Tycho Andersen <tycho.andersen@canonical.com>
Reported-by: Ross Boucher <boucher@gmail.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>