Unfortunately the kernel wakes up only one waiter even
if they waits with MSG_PEEK, so the introduced scheme doesn't work.

Reported-by: Mr Jenkins
travis-ci: success for Revert "zdtm: use a unix socket instead of a pipe to synchronizer processes"
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>