Some partition tables have partitions that are not aligned to a
cylinder boundary, which confuses the current geometry guessing logic.

Enhance the existing logic to ensure that we never reduce our guesses
for the number of heads or sectors per track, and add extra logic to
calculate the exact number of sectors per track if we find a partition
that starts within cylinder zero.

Signed-off-by: Michael Brown <mcb30@ipxe.org>