datapath-windows: Add Datapath.[ch] and OVS_USE_NL_INTERFACE CPP

datapath-windows: Add Datapath.[ch] and OVS_USE_NL_INTERFACE CPP

In this change we create peer to OvsIoctl.[ch] that would interface
with userspace using the netlink interface. The new files are called
Datapath.[ch]. We are not deleting OvsIoctl.[ch] yet. Datapath.[ch]
provides bare minimum functionality to register a pseudo device for
communication with userspace.

We also define a CPP called OVS_USE_NL_INTERFACE in the ovsext project.
This defines if the old interface (pre-netlink based) or the new
interface (netlink- based) should be enabled in the kernel. By default,
the value is 0 since the Datapath.[ch] are minimal. In the next change,
we'll turn it to 1.

Since Datapath.[ch] containes code copied from OvsIoctl.[ch], acking
the original author as well.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Co-authored-by: Guolin Yang <gyang@vmware.com>
Acked-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>