Add some of the missing parts to libkir.h.

link-time check for section overlaps.  (In order to avoid wasting
space in the executable image, .bss16 will overlap with the following
section, which is .text).

somewhat redundant, and also causes gas to complain when we include
the profiling code (which uses an i586 instruction).

userptr_add().

to the REAL_CODE interface.

Fix bug that caused over-allocation of .text16 and .data16 memory areas
by a factor of 16.

completely.

the only one we actually use).  This allows REAL_EXEC fragments to
contain proper references to constraints (e.g. "%w0"), rather than having
to force the use of specific registers.

Note that the "num_constraints" parameter is now completely obsolete, and
that we can probably reduce the syntax to something like
  __asm__ __volatile__ ( REAL_CODE ( "asm statements" )
                         : output constraints
                         : input constraints
                         : clobber );
which would look much more natural, and avoid the need to always specify
a clobber list.


Add userptr_t to libkir.h, to allow it to at least compile.

usual __{text,data}16 trick work with arrays.  gcc seems to accept the
__asm__ ( asmlabel ) only after the [] of the array declaration, not
before.

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).

Use .text16.data section with "aw" attributes, to avoid section type
conflicts when placing both code and data into .text16.

Add __from_{text16,data16}.

interrupt vectors, for example, will be easiest to handle if placed in
.text16.

.data16.  librm will need to supply "char *data16", i.e. the virtual
address of the start of .data16.

However, it's worth adding a .size directive so that objdump reports the
correct sizes for the code fragments.

advantage of the fact that we have to have a permanently-resident block
in base memory.