unix: add non block send and receive flavors for fd passing

These will be used by the notification subsystem.

It is important to note that based on our current knowledge the sending
/receiving of fds is an all or nothing scenario. The fds are actually
part of the control message instead of the `payload`. On the receiving
side, a reception of N fds will only yield a "read" count of 1 bytes on
reception.

Albeit we don't have to account for the partial send/receive, we have to
manage the EAGAIN/EWOULDBLOCK scenario off non-blocking socket.

The caller of these function must handle the following scenario:

  ret < 0 -> error
  ret == 0 (Nothing received/sent)
  ret > 0 -> success

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Iabf8de876991c9f1c5b46ea609f9af961c4a7ab9