We don't need the ANSI engine in mboot.c32.

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.

Make the syntax for chain.c32 more Linux-kernel-like.  This also makes
parsing easier, so it's a win all around.

Comma-space takes up too much horizontal space, since we
want to be able to comment properly.

Check the stub sources into the tree, even though they aren't used by
the build; they are still useful when experimenting with changing this
stuff.

dosmem and int13 can change during boot loader shutdown.  It
is therefore unsafe to rely on them retaining their value from
running in chain.c32 itself.  Instead, we have to install a
suitable stub which makes that particular determination at
runtime.  Fortunately that is not all that hard to do.

Support loading files below 0x7c00, as required, for example, by DOS
kernels.  Furthermore, pass the drive number in BL as well as DL.

With this, we can load the FreeDOS kernel with:

     chain.c32 hd0 -seg 0x60 -file /kernel.sys

Should really clean up the syntax before 3.70 final, though, and
provide a way to default to the syslinux device.

SS is segment 2, DS is segment 3; this header had them backwards.
Fortunately not too many things relied on having DS != SS.