GDB serial and backtrace support.

	GDB serial and backtrace support.

	* grub-core/kern/i386/realmode.S (real_to_prot): Reload IDT.
	(prot_to_real): Likewise.
	* grub-core/kern/i386/int.S (grub_bios_interrupt): Remove IDT reload.
	* grub-core/Makefile.core.def (backtrace): New module.
	(gdb): Likewise.
	* grub-core/gdb/cstub.c: New file.
	* grub-core/gdb/gdb.c: Likewise.
	* grub-core/gdb/i386/idt.c: Likewise.
	* grub-core/gdb/i386/machdep.S: Likewise.
	* grub-core/gdb/i386/signal.c: Likewise.
	* grub-core/lib/i386/backtrace.c: Likewise.
	* include/grub/backtrace.h: Likewise.
	* include/grub/gdb.h: Likewise.
	* include/grub/i386/gdb.h: Likewise.

	Also-By: Vladimir Serbinenko <phcoder@gmail.com>