Implement Richard's really clever idea about bell overload mode:

it's automatically deactivated by any keypress, so that command-line
beeps from (e.g.) filename completion don't suddenly stop occurring,
but it still provides a rapid response to an accidental spewing of a
binary to your terminal.

[originally from svn r2107]

unix/pterm.c

@@ -786,7 +786,7 @@ gint key_event(GtkWidget *widget, GdkEventKey *event, gpointer data)
 
 	ldisc_send(output+start, end-start, 1);
 	show_mouseptr(0);
-	seen_key_event = 1;
+	term_seen_key_event();
 	term_out();
     }