Support 32bit addresses >64k as valid on bios calls.
Move stack for dosboot and biosboot to >64 so stack doesn't hit data.
Use disk sector number passed by mbr code to select default partition
(the mbr code doesn't do this yet).
NB only biosboot and dosboot have been tested so far.
(changes approved by christos and fvdl)
little hacking by me. This isn't strictly identical to his 920312
release -- I've hacked it a bit -- but since we are taking over change
control it doesn't matter much.