Stages of restoring

A process of restoring has a few global synchronization points:

CR_STATE_FORKING	Create tasks and restore group leaders
CR_STATE_RESTORE_PGID	Restore group IDs. Group leaders should exist in this moment and all helpers should be alive
CR_STATE_RESTORE	Wait helpers and restore a most part of resources. If anyone segfauls, its parent gets SIGCHLD and notify criu about that
CR_STATE_RESTORE_SIGCHLD	Restore SIGCHLD handlers
CR_STATE_COMPLETE	All handlers were restored, origin processes can be resumed.