fairland: Tweak sea oil & mountain gold for simplicity

fairland: Tweak sea oil & mountain gold for simplicity

The functions mapping elevation to resources are piecewise linear,
except for set_oil(), which randomly adds 1 to sea oil, and
set_gold(), which is isn't linear for mountains, but very close.

Drop the random oil bit, and replace the non-liner gold function by a
linear one.  Both resources decrease at most by one point.

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