zdtm: fix race in test_waitsig for multithreaded tests

A signal can be handled by non-leader thread and sigsuspend
will not be woken up.

kill can send signals to a specified thread, so a futex is used for
synchronization.

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