fairland: Prefer placing islands away from the edge of the sphere

When an island gets placed too close to the edge of the sphere of
influence, its side facing the edge will likely be formed by the edge.
Looks unnatural, and can give a clue on the location of the other
continent.

Make place_island() prefer sectors away from the edge: instead of
picking one of the admissible sectors with equal probability, reduce
the probability as we get closer to the edge.

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