rst: Introduce fine-grained pgid-restore synchronization

We can restore task's pgid which is not equal to its pid,
only when the respective group leader is alive. To make
restore reliable we wait for all group leaders to restore
using separate restore stage.

It's better to optimize this -- each task has a pointer on
its group leader and waits for one to become such.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>