Consider when there is a double fork of helpers or zombies, e.g. when a
zombie has a session id which doesn't match its pid. If the child dies and
exits before the grandchild, the grandchild reparents to init, and when the
task dies init doesn't have it in the helper list, so init dies as well,
viz. the log below.

(00.118789) Add a helper 293 for restoring SID 293
(00.118792) Attach 294 to the temporary task 293
...
(01.394403)    294: Restoring zombie with 0 code
...
pie: Task 294  exited, status= 0
(01.434279) Error (cr-restore.c:1308): 12097 killed by signal 19
(01.434420) Error (cr-restore.c:1308): 12097 killed by signal 19
(01.450258) Switching to new ns to clean ghosts
(01.450324) Error (cr-restore.c:2138): Restoring FAILED.

Let's have the helpers reap their children before they exit to avoid this.

v2: block SIGCHLD when waiting on helpers so that it doesn't race with the
    SICGHLD handler
v3: * only helpers should collect their children, zombies can't have kids
    * don't double decrement nr_tasks in zombie case
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>