1 or 2 hard disks.
- int13: check how many disks first, and only return an error if DL exceeds
the number of disks (ignoring bit 7 of course)
- added drive number arg to get_hd_geometry, so that get_hd_geometry can
retrieve the numbers corresponding to hard drive 0 or 1
- hard_drive_post will now set up the EBDA area for drive 0 if it exists,
then for drive 1 if it exists.
