add get_free_chars() function

server/drivers/hd44780.h

@@ -35,6 +35,7 @@ MODULE_EXPORT void HD44780_num (Driver *drvthis, int x, int num);
 MODULE_EXPORT int  HD44780_icon (Driver *drvthis, int x, int y, int icon);
 
 MODULE_EXPORT void HD44780_set_char (Driver *drvthis, int n, unsigned char *dat);
+MODULE_EXPORT int  HD44780_get_free_chars(Driver *drvthis);
 
 MODULE_EXPORT void HD44780_backlight (Driver *drvthis, int on);
 MODULE_EXPORT void HD44780_output (Driver *drvthis, int state);