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

Update copyright lines and standardize the format.

--once now supported for extlinux; some infrastructure for syslinux
there as well, need implementation in all the various installers,
however.

For the disk-based derivatives, handle the default ADV that is part of
the image.

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.