Plane: prevent GPS speed spikes at startup from breaking airspeed cal

the filter can be affected by a huge GPS velocity spike
Plane: changed FBW min to 9 m/s

6 m/s is too dangerous for widely used planes
RC_Channel: prevent overwrite of memory on high RC_Channel constructor
Copter: allow CH6 tuning of sonar gain
AP_Compass: fixed a bug in the tilt compass calculation

The simplification applied a few months ago was incorrect