socket-util: drain_rcvbuf() for Windows.

Netlink sockets are created as blocking sockets. So, we can't
afford to remove MSG_DONTWAIT for Linux.

drain_rcvbuf() is currently called from netlink-socket.c and
netdev-linux.c. As of now, I don't see it being used for Windows.

Bug #1200865.
Reported-by: Len Gao <leng@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>