fairland: Make actual island sizes fair

fairland: Make actual island sizes fair

The previous commit reduced the difference in island size within the
same batch of islands to at most one.  Eliminate the remaining
difference by shrinking the bigger islands by one sector.

This invalidates the precomputed exclusive zones, so recompute them.

fairland-test needs a tweak to avoid loss of test coverage.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>