This is a fix for my earlier "Remove some hardcoded dependencies between fdm,
viewer and acmodel" patch. The problem was discovered when testing the
wrightFlyer.
controls in the cockpit vs. which wheels they apply to. FlightGear now
sets /controls/gear/brake-left, /controls/gear/brake-right, and
/controls/gear/brake-parking. It should be up to the FDM to sort out
which wheels under which circumstances are affected by these controls
and ultimately what happens to the physical motion of the aircraft.