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

@@ -1294,6 +1294,7 @@ 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);