seize: detach from external processes before killing dumped processes (v2)

When we kill a container, all processes from its pidns are killed by
SIGKILL, but we don't expect that someone from the freezer cgroup will
be killed too if it was not dumped.

(00.468446) Error (seize.c:439): Unexpected child 79162
(00.468489) Error (seize.c:440): BUG at seize.c:440

This situation is posiable, if someone enters into pidns by setns.

In this patch, we deatches from extra processes before killing dumped
processes. In this case, we are not get signals if someone is killed.

v2: use process_to_wait insread of adding a new var

https://jira.sw.ru/browse/PSBM-43795


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