restore: split finalize_restore into two stages

First stage of finalize_restore unmaps the restorer blob, second stage
detaches from processes. After first stage process tree is completely
restored and processes are ready to continue run through sigreturn.
This splitting allows us to execute something between these stages (e.g.
restore freezer cgroup state).

Signed-off-by: Evgeniy Akimov <geka666@gmail.com>
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Acked-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>