libcg: scan running tasks at start time to classify existing pids

libcg: scan running tasks at start time to classify existing pids

Add routine to scan rules.conf file and move matching running tasks
in /proc/pid/* into configured control groups. Then at init time
we can move running tasks into the correct control group.

Expose this routine via libcg so other applications can use it
to classify existing applications after creating control groups.

CC: Jan Safranek <jsafrane@redhat.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Jan Safranek <jsafrane@redhat.com>