Use fake netlink socket for agent<=>module communication.

Communication between vRouter kernel module and agent has been encapsulated
(back) in Netlink. For that purpose, fake implementaion of Netlink has been
provided on BSD (non-systemwide). This implementation contains only structures
and defines that are used by Contrail's source code, not more.
This measure has been taken to limit number of changes and differences
between implementations of the protocol (vRouter<->agent), as due to some
of characteristics of Netlink protocol being used by the protocol additional
structures would have to be provided to provide missing functionality.

Change-Id: I4a34b8dc6abc59daa2db8f1863b9e54658255ce1