Obtain a list of subsystems from /proc/cgroups, and ignore hierarchies
that are not bound to any of them (especially the 'systemd' hierarchy:
http://www.freedesktop.org/wiki/Software/systemd/PaxControlGroups ).
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>