[CRIU] [PATCH cr 0/11] Restore SID (v4)

[Andrey Vagin]

We need to reconstruct a processes tree, because a sid can't be set,
it can be only inherit.

Thre fundamental cases exist:
* A session leader is dead
  - add a fake task
* A task is detached to init
  - add a fake task and connect it to the session leader
* A parent forked a child, then changed own sid
  - determining a sid with which the parent was born.

v2: fix according with Cyrill's comments.
v3: fix according with Pavel's comments.