The purpose of this patch is to limit the search
of pids to those in containers by looking at first
in the /cgroup/<name>/tasks  when --lxc or --names options
are specified by user.
The idea is to speedup the output when only few
container names are specified while the machine
is running with many processes.
Signed-off-by: Michel Normand <michel.mno@free.fr>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>