Adds pickup truck

Pretty much the same wheelbase as the Car, with a bigger engine and the
seats swapped for trunk capacity.  Smaller version of the flatbed.
Pickup truck spawns

Gives a chance for the pickup to spawn on highways, bridges, and
replacing the flatbed at farms.
Moves pickup truck to avoid merge conflict

Because otherwise the mirror PR might well get mixed in with the shift
from the pickup.
Merge pull request #9479 from KA101/pickup

Pickup truck