Cargo body improvement

Closes #1659. Implemented it as suggested by robn's comment in that issue. 

- cargo takes damage on collision unless with ship carrying cargo scoop.
- cargo self destructs after some time (default 24 h).
- cargo self destruct can be set or disabled from Lua.
- since cargo is very light (1t) remove lower cap on collision damage.

To test the Lua API part:

```lua
h = import("Equipment").cargo.hydrogen
import("Game").player:SpawnCargo(h)    -- default self destruct 24 h
import("Game").player:SpawnCargo(h,30) -- shorter self destruct, 30 s
import("Game").player:SpawnCargo(h,0)  -- no selfdestruct
```