stream-fd: Merge stream-fd-windows and stream-fd-unix.

There was not much difference between the two files after moving
all of the Windows socket HANDLE polling functionality to poll-loop.c.
So merge them together.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>