Terminate UNDI and PXE before launching the payload to avoid problems

	Terminate UNDI and PXE before launching the payload to avoid problems
	with DMA.

	* grub-core/commands/boot.c (grub_loader_noreturn): Rename to ...
	(grub_loader_flags): ... this. All users updated.
	(grub_loader_boot): Check for GRUB_LOADER_FLAG_NORETURN.
	* grub-core/loader/i386/pc/pxechainloader.c (grub_cmd_pxechain): Mark
	loader as GRUB_LOADER_FLAG_PXE_NOT_UNLOAD.
	* grub-core/net/drivers/i386/pc/pxe.c (grub_pxe_shutdown): New
	function.
	(grub_pxe_restore): Likewise.
	(fini_hnd): New var.
	(GRUB_MOD_INIT): Register shutdown hook.
	(GRUB_MOD_FINI): Shutdown and unregister shutdown hook.
	* include/grub/loader.h (GRUB_LOADER_FLAG_NORETURN): New const.
	(GRUB_LOADER_FLAG_PXE_NOT_UNLOAD): Likewise.
	(grub_loader_set): Rename second argument to flags.