Go back to occupying just a single sector, reverting r1.17 - r1.20.

Go back to occupying just a single sector, reverting r1.17 - r1.20.
Taking over the sector following the MBR causes problems on some
machines, and the actual gains are fairly small in terms of how
the space is presently used.

Since we need a number of further features (eg. handling extended
partitions) that can't be readily accommodated in the basic boot0
design anyway, rather choose to implement the additional stuff
separately and concentrate on compatibility rather than features
here.