For example, if a zombie has a helper that sets up its session id, the
zombie will be reparented to the init task, which will then potentially get
a SIGCHLD for a task which isn't its direct child zombie, which we didn't
handle. Instead, let's find all the zombies for the init task, in case they
get reparented this way.

v2: only the zombies need to be recursively collected, helpers wait on
    their children before they exit and will never be reparented
v4: the root task waits all zombies
Reported-by: Tycho Andersen <tycho.andersen@canonical.com>
Cc: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>