pstree: Link pid and pstree_item

Add a link from task's pid to pstree_item.
Threads have this link set in NULL.

travis-ci: success for Make pstree_item::pid allocated dynamically
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>