Define close_file slightly tighter:

       - It is permitted to call close_file with a zero filehandle
       - close_file zeros SI

Fix the handling of the new API in SYSLINUX.  It appears file_left was
never actually initialized; it simply didn't matter since the old code
would either trip on reaching the end of the FAT chain or count the
data it needed.  Now operating according to spec.

Support the new getfssec API which returns number of bytes read in
SYSLINUX and EXTLINUX.  Untested so far, but it's a reasonably easy
change.

Finally scrub the code and get rid of the old uses of DX:AX for file
size (rather than EAX).

Support putting virtual kernels in high memory instead of using a
dedicated segment for it.  This both reduces the low memory footprint
by 64K, and allows for functionally unlimited labels (tested with over
a hundred thousand.)

There was a comment which applied to SYSLINUX that had been copied
verbatim to the other derivatives; fix.

The file display and graphics routines used their own file management
since the old getc code did not nest.  The new one does (necessary to
support INCLUDE), so use the same routines and get rid of the ad hoc
buffer management.

Change the command line limit to 2047 characters, which is also the
current kernel limit.  As a result, shuffle around the memory layout
to make the larger buffers fit.

TODO: consider moving vgafontbuf to getcseg, and try to make do with
fewer command line-sized buffers.

Remove the last few uses of getcbuf (to extend trackbuf for shuffle
descriptors, and for temp storage during pxelinux config file search.)
As a result, change the way RBFG_brainfuck is handled in pxelinux (at
some point it's probably not worth maintaining anymore, as RBFG is no
longer widely used); add a .bss2 section for the region between the
RBFG zone and the stack.

Correct a bug that caused EXTLINUX to crash when accessing an empty
file.

Update copyright lines and standardize the format.