unix: Wait a peer using task_st futex

Use task_st futex notifier instead of per-socket.
The step to make file opening use the only futex.

v2: Use internal bound and listen states instead of generic

travis-ci: success for Rework file opening scheme to make it asynchronous (rev5)
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>