Light/medium truck rebalance

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.
Spawn tweak.
Merge pull request #10409 from KA101/flatbed

Flatbed rebalance (buff + decreased spawn)