Avoid touching memory at address zero when going from a background to
a non-background.

Allow the initrd to be specified on a separate line from command-line
options (append).  This apparently can help certain tools.

Really, really try to avoid stepping on disabled entries...

Default to \n -> \r\n conversion in the ANSI and xserial modules,
rather than relying on escape codes to make that behave.  This
effectively means that as far as the serial port is concerned, this
behavior cannot be turned off (with less than having the xserial state
machine interpret this sequence) and the escape code *should not* be
used.

That is fine for our applications, though.

Begin the process of centralizing configurables.

Improve "make install" and "make netinstall"; add "make
extbootinstall".

Move source files out of the root directory; the root is a mess and
has become virtually unmaintainable.  The Syslinux core now lives in
core/; the Linux and generic utilities has moved into utils/, and
copybs.com has moved into dos/; it had to go somewhere, and it seemed
as good a place as any.

Use "-nostdinc -iwithprefix include" to prevent the inclusion of
system header files, but still permitting *compiler* header files.

Break off execute() into its own source file, with the intent of being
able to re-use it for a CLI module.

Allow MENU EXIT to "exit" to an arbitrary menu.  This is really just a
variant of "MENU GOTO", but it allows displaying an exit marker.