For moving to a single transport socket scheme, we need to be able to differ
fds in receive queue from each other. Add a fle pointer as identifier for that.

v2: Rebase on compel
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>