seize: don't use wait4() to check whether or not a task is traced

If a task is stopped, waitpid() will return its status, but
wait4() in seize_wait_task will hang forever, because a stopped
state is reported only once.

v2: don't like a file object

Reported-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>