netlink-socket: User mode event read for Windows.

User mode sends down three distinct Read ioctl commands for Events, Packet
Reads and Dumps. In case the Packet Read socket can not be distinguished a
Set function will be provided.

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