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.

Change method of bind mounting /proc/<pid>/net onto /proc/net, to
avoid error "cannot mount block device /proc/<pid>/net read-only".

Check that user is root. Check that container name is specified
before calling 'exec'.

Update the help information.

Print error messages and help information to stderr.

Make indentation consistent.
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>