datapath-windows: add infrastructure for supporting netlink

datapath-windows: add infrastructure for supporting netlink

In this change, we define netlink families and commands supported
by the Windows datapath. Only the control family and one command
is supported today to get the PID.

We also implement the 3 ioctls: read, write and transact. This is
bare minimum and can be used to implement the equivalent of a
recv, send, and send-recv in userspace netlink. This interface
is subject to change as iron out the details of the user-kernel
interface for operations around packet receive, events, dump, etc.

We also turn on OVS_USE_NL_INTERFACE to 1 in the ovsext project.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/18
Signed-off-by: Ben Pfaff <blp@nicira.com>