* fhandler.cc (fhandler_base_overlapped::wait_overlapped): Handle

* fhandler.cc (fhandler_base_overlapped::wait_overlapped): Handle
overlapped_fallback error condition like other error conditions.  Set res
carefully and specifically for each condition rather than resorting to a
default.
(fhandler_base_overlapped::write_overlapped): Preserve errno in
overlapped_fallback condition.  Correct write_overlapped_fallback to avoid
inappropriate looping.
(fhandler_base_overlapped::write_overlapped_fallback): Add some more comments.