Add an '--active' option that lists active containers by searching
cgroups. (Otherwise, the directories in /var/lib/lxc are listed.)
Modify the cgroup search to only use hierarchies that contain one
or more subsystems. When searching, if a hierarchy contains the
'ns' subsystem, do not append '/lxc' to the parent cgroup.

Add a '--help' option that prints the command syntax.

Print error messages and help information to stderr.

Update the documentation.
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>