Flatbed rebalance (buff + decreased spawn)

This gives flatbeds the cargo-space structure instead of their trunks, but noticeably decreases their spawn in favor of the smaller pickup truck.  In particular, malls, civilian housing, the evac shelter, and most large structures will spawn pickups instead of flatbeds.

(The flatbed truck is a commercial-grade cargo hauler, and not something most people keep as a personal vehicle.)

Flatbeds can still be found in the warehouse and public works, and rarely at the mall, office tower (deliveries) and generic parking lots.