Cygwin: FIFO: synchronize the blocking mode of a writer's pipe

The blocking mode of the Windows pipe underlying a writer is set to
match that of the writer itself when the latter is opened.  Define
fhandler_fifo::fcntl to keep the pipe and the writer in sync if the
blocking mode is changed via fcntl.

winsup/cygwin/fhandler.h

@@ -1300,6 +1300,7 @@ public:
   int open (int, mode_t);
   off_t lseek (off_t offset, int whence);
   int close ();
+  int fcntl (int cmd, intptr_t);
   int dup (fhandler_base *child, int);
   bool isfifo () const { return true; }
   void set_close_on_exec (bool val);

winsup/cygwin/fhandler_fifo.cc

@@ -891,6 +891,22 @@ fhandler_fifo::close ()
   return fhandler_base::close () || ret;
 }
 
+/* If we're a writer, keep the nonblocking state of the windows pipe
+   in sync with our nonblocking state. */
+int
+fhandler_fifo::fcntl (int cmd, intptr_t arg)
+{
+  if (cmd != F_SETFL || !writer)
+    return fhandler_base::fcntl (cmd, arg);
+
+  const bool was_nonblocking = is_nonblocking ();
+  int res = fhandler_base::fcntl (cmd, arg);
+  const bool now_nonblocking = is_nonblocking ();
+  if (now_nonblocking != was_nonblocking)
+    set_pipe_non_blocking (get_handle (), now_nonblocking);
+  return res;
+}
+
 int
 fhandler_fifo::dup (fhandler_base *child, int flags)
 {