[Freeciv-Dev] small trade patch (PR#198)

[Peter Schaefer <schaefer@xxxxxx>]

 The Trade Weighting has to about as large as the Shield Weighting,
 otherwise the AI will build Barracks to create veterans in cities
 with only 1 shields production.

 (On the other hand, about 2 trade is needed to buy one unit of
production.. .)

The patch 
- changes TRADE_WEIGHT from 8 to 12
- makes some init functions use TRADE_WEIGHT instead of 8
-renames a single letter variable to a more telling name.