xnu support

	* conf/i386-efi.rmk (kernel_mod_HEADERS): added i386/pit.h
	(pkglib_MODULES): add xnu.mod
	(xnu_mod_SOURCES): new variable
	(xnu_mod_CFLAGS): likewise
	(xnu_mod_LDFLAGS): likewise
	(xnu_mod_ASFLAGS): likewise
	* conf/i386-pc.rmk: likewise
	* conf/x86_64-efi.rmk: likewise
	* include/grub/efi/efi.h (grub_efi_finish_boot_services): 
	new declaration
	* include/grub/i386/macho.h: new file
	* include/grub/i386/xnu.h: likewise
	* include/grub/macho.h: likewise
	* include/grub/machoload.h: likewise
	* include/grub/x86_64/macho.h: likewise
	* include/grub/x86_64/xnu.h: likewise
	* include/grub/xnu.h: likewise
	* kern/efi/efi.c (grub_efi_finish_boot_services): new function
	* kern/efi/mm.c (MAX_HEAP_SIZE): increase
	* loader/i386/efi/xnu.c: new file
	* loader/i386/pc/xnu.c: likewise
	* loader/i386/xnu.c: likewise
	* loader/i386/xnu_helper.S: likewise
	* loader/macho.c: likewise
	* loader/xnu.c: likewise
	* loader/xnu_resume.c: likewise
	* util/grub-dumpdevtree: likewise
	* include/grub/i386/pit.h: include grub/err.h
	(grub_pit_wait): export
	* util/grub.d/30_os-prober.in: support Darwin/Mac OS X