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>