Oops, the previous revision (1.35) broke booting from floppies
because the buffers we use could end up spanning a 64k boundary.
Unfortunately it causes too much bloat (228 -> 72 bytes free) to
just reinstate the old malloc() function.

Instead, define a structure that contains all 4 buffers which must
not cross 64k boundaries. We allocate a 64k-aligned instance in
main() using the magic that was in the old boot2 malloc() function.
This brings the free space down to 168 bytes, but that is still
better than it was before revision 1.35 (136 bytes).

Reported by: Mike Brancato <funnyguy@digitalsmackdown.net>
Pointy-hat to: iedowse