prioritizing truck tasks

When assigning vehicle tasks, have the algorithm prioritize using Haul Truck (dump/tank) for lose/fluid cargos. Since the haul trucks cannot transport unit cargoes, this would theoretically keep more unit-capable trucks free to haul those, while the haul trucks handle the other cargoes more efficiently.

Haul and Tank trucks are specific to T3 vehicles. The T1 and T2 vehicles exchange their setup as needed. So an empty T2 Haul Truck will accept a task transporting Units, go to where it needs to be, turn into a Unit Truck, load the Units, and go. Saying haul trucks cannot transport unit cargoes is only true for T3.