Cygwin: pty: Set input_available_event only for cygwin pipe.

- cat exits immediately in the following senario.
    1) Execute env CYGWIN=disable_pcon script
    2) Execute cmd.exe
    3) Execute cat in cmd.exe.
  This is caused by setting input_available_event for the pipe for
  non-cygwin app. This patch fixes the issue.

winsup/cygwin/fhandler_tty.cc

@@ -386,7 +386,8 @@ fhandler_pty_master::accept_input ()
 	}
     }
 
-  SetEvent (input_available_event);
+  if (write_to == get_output_handle ())
+    SetEvent (input_available_event);
   ReleaseMutex (input_mutex);
   return ret;
 }