fairland: Simplify search for next wilderness to elevate

fairland: Simplify search for next wilderness to elevate

To find the wilderness sector to elevate next, elevate_land() searches
the non-mountain, non-capital sectors of the island for one that
maximizes a function of its distance to mountains and to sea.

The search ignores already elevated sectors in a less than obvious
way: 1. it never picks a sector where the function yields -INFINITY or
less, and 2. when elevating a wilderness, its (cached) distances get
reset to values that make the function return a more negative value.

Use a more direct check of "not yet elevated": elevation is still the
initial -INFINITY.

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