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/include/gpxe/uaccess.h

@@ -0,0 +1,24 @@
+#ifndef _GPXE_UACCESS_H
+#define _GPXE_UACCESS_H
+
+/**
+ * @file
+ *
+ * Access to external ("user") memory
+ *
+ * gPXE often needs to transfer data between internal and external
+ * buffers.  On i386, the external buffers may require access via a
+ * different segment, and the buffer address cannot be encoded into a
+ * simple void * pointer.  The @c userptr_t type encapsulates the
+ * information needed to identify an external buffer, and the
+ * copy_to_user() and copy_from_user() functions provide methods for
+ * transferring data between internal and external buffers.
+ *
+ * Note that userptr_t is an opaque type; in particular, performing
+ * arithmetic upon a userptr_t is not allowed.
+ *
+ */
+
+#include <bits/uaccess.h>
+
+#endif /* _GPXE_UACCESS_H */