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.)

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.

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.

Update copyright lines and standardize the format.

Initial support for the auxillary data vector, a storage mechanism
intended to support boot-once and similar features.

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.

Remove unused variable BufSafeSec; it was identical to BufSafe in
every way.

At the moment, BufSafe* are always constants; consider if they can be
swapped out for constants or if we want to retain them for flexibility.