Allow to set lawlessness for custom systems

This was inspired by a comment by @impaktor in #3165

You can now use ```lawlessness(fixed_value)``` in custom systems. If you don't, the lawlessness will be random as before (so now bump needed as long as we don't use it in custom systems).

To test:
- Add ```:lawlessness(f(1,100))``` to Sol
- Start game at Earth
- In Lua console: ```print(Game.system.lawlessness)``` (beware of rounding errors ;))