Thought as much

Locking things to engine_on looked like it did it.  Too easy: broke
bicycles.

This allows muscle-powered vehicles to move, and locks out installing a
muscle engine if any other engine exists in the vehicle.  That's to
prevent jokers from installing hand rims in their hatchback and
exploiting the muscle-power bypass.

Only thing keeping this WIP is my not being able to find a way to
indicate that last lockout in the veh_interact UI.  Will look into it.