Plan9 support.

	Plan9 support.

	* Makefile.util.def (libgrubmods): Add
	grub-core/partmap/plan.c.
	* docs/grub.texi: Notice Plan9 support.
	* grub-core/Makefile.core.def (plan9): New module.
	(part_plan): Likewise.
	* grub-core/loader/i386/pc/plan9.c: New file.
	* grub-core/partmap/plan.c: Likewise.
	* include/grub/msdos_partition.h (GRUB_PC_PARTITION_TYPE_PLAN9): New
	define.
	(GRUB_PC_PARTITION_TYPE_LINUX_SWAP): Likewise.
	* include/grub/mm.h (grub_extend_alloc): New inline function.