datapath-windows: Add Netlink buffer management APIs.

In this change we have introduced buffer mgmt apis which will be
used while creating netlink messages. The basic functionality provided
by apis is on similar lines to ofpbuf in userspace with an exception
that it will not do run time buffer reallocation.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Tested-by: Ankur Sharma <ankursharma@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/37
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean at cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>