Support submenus.

	Support submenus.

	* grub-core/commands/menuentry.c (grub_normal_add_menu_entry): New
	parameter submenu. All users updated.
	* grub-core/normal/main.c (free_menu): Rename to ...
	(grub_normal_free_menu): ... this. Made global.
	* grub-core/normal/menu.c (grub_menu_execute_entry): Open new context
	if requested.
	* grub-core/normal/menu_entry.c (screen): New field submenu.
	(make_screen): Set submenu.
	(run): Open new context if requested.
	* include/grub/menu.h (grub_menu_entry): New field submenu.
	* include/grub/normal.h (grub_normal_free_menu): New proto.