(was "files: Wait transport_fd before sending a fd to peer"
and "pstree: Add task_st futex")

We are going to move to the the single per-task bit field
for notifications about file opening events. Introduce
pstree_item::task_st for that.

v5: Add FDS_EVENT_BIT description.

v2: Do not wait until a peer's socket is created
as it's guarantied by we create it before CR_STATE_FORKING.

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>