A unix socket can be bound to itself, so we need only one socket
instead of a pair of pipe.

If we use a socket, we can peek a message and if it isn't for us
we don't need to send it back.

Currently a process reads message from pipe, if it isn't for him,
it sends it back and sleeps for a random timeout. In a worst case
each waiter will read and send a message back, before the right
one will get it.

travis-ci: success for series starting with [1/3] zdtm: call test_init() before doing anything
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>