Fix drag when engine is shut down, by avoiding negative RPM.

Fix drag when engine is shut down, by avoiding negative RPM.

The Propeller class ignored negative RPM but still returned a torque
value, which ratcheted up a higher and higher negative RPM until drag
overwhelmed the aircraft.

In reality, the propeller should windmill at a reasonable postive RPM,
introducing a constant drag on the aircraft -- the propeller should
*not* stop unless the plane is flying very slowly.  That's a future
project.