Trader should have different treshhold level

```
Game version: BETA

Right now trader ships turn around after having received a fixed amount of
damage (50%?).

DOS IG has a differentiated behavior:

Normal trader: a few shot are enough
A trader with uninfected people trying to leave an infected planet: about 50%
the unknown smuggler trader: maybe 80%

and so on

This is a far more realistic behavior. Why should a normal trader risk his life
when being attacked :)

```

Original issue reported on code.google.com by `norbert....@gmail.com` on 9 Jun 2012 at 9:42