fairland: Replace distance_to_land(), distance_to_sea()

fairland: Replace distance_to_land(), distance_to_sea()

distance_to_land() searches for closest land sector, and
distance_to_sea() for the closest sea sector.  We already have a more
efficient alternative: the breadth-first search recently added for
spheres of influence can precompute these distances.  Put it to use,
and retire distance_to_land() and distance_to_sea().

distance_to_what() could now be simplified.  Don't bother, because
it'll soon be deleted entirely.

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