lxc-attach functionality reads /proc/init_pid/cgroup to determine the cgroup
of the container for a given subsystem. However, since subsystems may be
mounted together, we want to be on the safe side and be sure that we really
find the correct mount point, so we allow get_cgroup_mount to check for
*all* the subsystems; the subsystem parameter may now be a comma-separated
list.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>