datapath-windows: define a version number in DP extensions

We need a version number for the datapath interface extensions that
we can leverage at some point.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
netlink-socket.c: add support for do_lookup_genl_family on Windows

In this patch, we add support for querying the genl family id for any
family supported by the OVS kernel datapath. On platforms that support
netlink natively, the operating system assigns a family ID, and the
OS netlink infrastructure supports querying the family ID by name.

In case of Windows, since OVS datpath provides the netlink support,
it is not necessary to make a call into the kernel. Returning a
family ID that is consistent between the userspace and kernel
is sufficient. Once there is code to support netlink message parsing
as well as constructing netlink messages, we can make a call into
the kernel, but that in itself may not buy anything more than this
approach.

This patch is a precursor to make progress of the other commands.
The next hurdle is to support nl_lookup_genl_mcgroup().

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