Add extentions to the standard datapath interface

Add extentions to the standard datapath interface

The datapath interface defined in odp-netlink.h needs some extensions
that are platform dependent. Some examples are the name of the communication
device on Windows and a set of commands that are specific to Windows.

In this change we define a datapath-windows/include/OvsDpInterfaceExt.h
to include any platform specific interface extensions.

OvsDpInterfaceExt.h is in turn included in odp-netlink.h ONLY for _WIN32.
This approach was chosen to avoid including OvsDpInterfaceExt.h directly
although the latter approach is as good as the former.

Also, we define three ioctls in OvsDpInterfaceExt.h:
read:     provides an output buffer (mimics a recv)
write:    provides an input buffer (mimics a send)
transact: provides an input and optionally an output buffer.
          (mimics a send followed by recv)

Signed-off-by: Nithin Raju <nithin@vmware.com>
Co-Authored-by: Ben Pfaff <blp@nicira.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>