The $PnP probe has been found to lock up at least one machine for
reasons unknown.  Drop it; instead, burn the extra few bytes in the
bootsector to save away ES:DI for later restore.

Change DriveNo to DriveNumber in ISOLINUX, to match the other
derivatives.

Clobber "mov cs,ax" with "nop nop" even though that adds a byte to the
source; it's a lot easier to comprehend while debugging.

Fix shuffle_and_boot_rm, which generated an invalid trampoline; this
was used by linux.c32 among other things.

Update the memory layout specification.  Remove NASM 0.98.38 bug
workarounds, rebalance the bss sections, and introduce the new section
.adv which contain the auxilliary data vector.

Fix memory overwrite bug, specifically, the use of BP after it might
have gotten clobbered.  Since at the point of use, BP is supposed to
be == SP, we can just add "mov bp,sp".

The SYSLINUX internal API closes a file automatically upon reading
to block EOF (not necessarily file EOF!)  Thus, it's fairly likely
we'll get SI == 0 (closed file) passed to close_file at some point
or another.

Just plain admit it, and make close_file accept SI == 0 by spec.

Add a "close_file" function and hook it up to the long-standing comboot
API function.  At the moment, all it does is free up internal
resources; in particular, for PXELINUX it should also really send an
ERROR packet to the server to terminate the connection on that end.