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

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.

Settle on a final implementation of the NTLDR support.

Don't install the swap stub when we are already the primary drive,
even if -swap is specified.