Added GrindsUnit trait for RA2:YR-style Grinder structure logic.

Added GrindsUnits trait:
```
int RefundPercent
bool RefundPassengers
string GrindSound
```

Code style cleanup in Passenger.cs

Need a way to show normal cursors (non-cargo) even if we have `Cargo:`

Test case in RA with POWR (will remove after testing): Accepts inf and vehicles (APCs),

Also useful for #5910 as the Soviet (later Yuri) Cloning Vat had this feature for infantry.
You should also fix/update the comment about the (now fixed?) EjectOnDeath oddity.

I will hold off reviewing this until these have been fixed.