Add the concept of a "user pointer" (similar to the void __user * in

the kernel), which encapsulates the information needed to refer to an
external buffer.  Under normal operation, this can just be a void *
equivalent, but under -DKEEP_IT_REAL it would be a segoff_t equivalent.

Use this concept to avoid the need for bounce buffers in int13.c,
which reduces memory usage and opens up the possibility of using
multi-sector reads.

Extend the block-device API and the SCSI block device implementation
to support multi-sector reads.

Update iscsi.c to use user buffers.

Move the obsolete portions of realmode.h to old_realmode.h.

MS-DOS now boots an order of magnitude faster over iSCSI (~10 seconds
from power-up to C:> prompt in bochs).

src/arch/i386/core/pic8259.c

@@ -6,7 +6,7 @@
 
 #include <etherboot.h>
 #include "pic8259.h"
-#include "realmode.h"
+#include "old_realmode.h"
 
 /* State of trivial IRQ handler */
 irq_t trivial_irq_installed_on = IRQ_NONE;