* autopilot: hold indicated altitude, not absolute altitude.
* FDM: increase the static and dynamic friction of the nose gear to
improve steering on the ground.
* FDM: restrict nose gear steering to 58.5 degrees when the pushback
truck is not connected, 70 degrees when connected (in reality it was
possible to disconnect the steering and rotate the nose gear 90
degrees by hand, but the shared model for the pushback truck allows
only a hardcoded 70 degrees).
