parasite: stop a parasite daemon before dumping threads

The parasite daemon set up SIGCHLD handler, but for dumping threads we
use parasite-trap. While doing this the sigchild handler notices the
CHLD arriving on the thread trap, emits an error

(00.020292) Error (parasite-syscall.c:387): si_code=4 si_pid=3485 si_status=5

but wait() reports -1 (task is not dead, just trapped) and handler just exits.

Let's stop a parasite daemon before dumping threads.

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