cap maximum number of police GoodsTraders

## Description ##

This prevents placing too many `ispolice` Goodstraders, so with the parameters I've chosen, `maxPolice=1`, one can not have two police goods traders, since it will skip placing the ad if it turned out to be a police again.

So this just removes the 

- total of 2 goodstrader, both are police

But the following will still be possible:

- total of 1 goodstrader, who is police
- total of 1 goodstrader, who is not police
- total of 2 goodstrader, none are police
- total of 2 goodstrader, one is police

(I just wraped the code in an if-statement, and re-indented.)

Closes #3158 

## To do in the future ##

I noticed that _number_ of GoodsTraders or _fraction_ of them that will be police is not affected by `lawelssness`, only fine is. Nor is _price_ affected. Before that is fixed I think `lawlessness` needs to be able to be set for custom system, since I think Sol should be close to 0, and not the random non-zero value it now has.