crtools: link pstree_item-s in a tree (v3)

because they describes a process TREE.

It's usefull, when we dump tasks from another pid namespace,
because a real pid is got from parasite. In previous version
we need to update pid in two places one is in a pstree_item and
one is in a children array.

A process tree will be necessery to restore sid and pgid,
because we should add fake tasks in a tree. For example if
a sesion leader is absent.

v2: fix rollback actions
v3: fix comments from Pavel Emelyanov
    * add macros for_each_pstree_item
    * and a few bugs

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>