Before this patch crtools freezes processes and if something is changed,
it unfreezes all processes and starts again from the beginning.

If if are going to dump fork-bomb, this method doesn't work. Because a
big tree is always changed.

We don't need unfreeze processes, which have been frozen and this patch
does that.

This patch uses depth-first search (DFS) for traversing a process tree.

A root task is frozen at first turn, than a child is frozen, than a
child of child and so on.

When all children of one process are frozen, criu reads the list of
children again and check that nothing changed. This processes continues
until all of them will not be frozen. Afte that a new child can not be
appeared, because all children for children are frozen too.

v2: add comments in code
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>