Added RefundsUnits 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.

**Updates:** Please read the comments.
Could need a comma or two: `If the entering actor has cargo, should those actors be refunded, too?`