netlink-socket.c: implement get pid support on Windows

netlink-socket.c: implement get pid support on Windows

To verify if the netlink support in the kernel works, I updated
the netlink-socket.c code to get the PID for a given device
descriptor.

In the existing code, userspace sets the PID, which will not be
unique across different processes. So, it is better for the
kernel to generate the PID and give it back to userspace.

dpif-linux.c was ported to Windows (similar to Alin's change in
the cloudbase repo) and was able to exercise the code changes
in netlink-socket.c to read the PID. dpif-linux.c changes are
not being checked in.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Alin Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/18
Signed-off-by: Ben Pfaff <blp@nicira.com>