* dtable.cc (dtable::select_write): Add missing argument to debug_printf.

* fhandler.cc (fhandler_base_overlapped::setup_overlapped): Explicitly set
io_pending to false.
(fhandler_base_overlapped::has_ongoing_io): Call GetOverlappedResult to force
completion of I/O.
(fhandler_base_overlapped::wait_overlapped): Rewrite to correctly deal with
nonblocking reads and to make more race proof.
(fhandler_base_overlapped::raw_write): Deal with new enum values.
(fhandler_base_overlapped::raw_read): Ditto.  Don't deal with ongoing I/O here
since it makes no sense in the read context.
* fhandler.h (enum wait_return): Add overlapped_unknown,
overlapped_nonblocking_no_data.
* pipe.cc (pipe): Add debugging output.

winsup/cygwin/fhandler.h

@@ -578,8 +578,10 @@ class fhandler_base_overlapped: public fhandler_base
 protected:
   enum wait_return
   {
-    overlapped_success = 0,
+    overlapped_unknown = 0,
+    overlapped_success,
     overlapped_signal,
+    overlapped_nonblocking_no_data,
     overlapped_error
   };
   bool io_pending;