ArduPlane: move VehicleClass handling to AHRS frontend

.... and renaming the enumeration while we're at it

ArduPlane/system.cpp

@@ -377,7 +377,7 @@ void Plane::startup_INS_ground(void)
 
     ahrs.init();
     ahrs.set_fly_forward(true);
-    ahrs.set_vehicle_class(AHRS_VEHICLE_FIXED_WING);
+    ahrs.set_vehicle_class(AP_AHRS::VehicleClass::FIXED_WING);
     ahrs.set_wind_estimation(true);
 
     ins.init(scheduler.get_loop_rate_hz());