restore: wait while restroring pgid (v2)

Pgid leader should become such before any other task tries
to enter this group (with setpgid). Thus we introduce a yet
another global sync point -- before it all pgid leaders call
setpgid after it all the others do it.

v2: wait while helpers restored pgid

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