Toggle engines

Adds ability to toggle individual engines in vehicle control menu, when more than one engine present. Choice brings up submenu listing all engines. Velocities, noise and fuel usage update accordingly.
If velocity after toggling is higher than the new safe velocity, the cruise control speed is lowered.

~~TODO: make radiators only work if the engine they are attached to is enabled~~
Yep, that function gets the user to select an engine index in the engines vector. Its called from control_engines, which handles disabling the engines. It calls toggle_specific engine to turn the engine on/off.