zdtm: rework a synchronization scheme

Currently test_daemon() sends a signal to a parent process,
when a test completed a prepartion stage. But it doesn't work,
if a child changed uid and gid, because it doesn't have enough
permissions in this case.

This patch reworks a synchronization scheme to use a futex
instead of sending signals.

Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>