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
Copter: revert to 3 element sonar's mode filter
Copter: surface tracking improvement

Separate correction speeds for pilot input vs altitude-above-ground
errors to allow slower correction to sonar signals.
Change SONAR_GAIN parameter to be altitude error -> desired speed.  This
means it should be set 10x larger than before.
Replace hard-coded sonar cut-off percentage of 70% with
SONAR_RELIABLE_DISTANCE_PCT.
Reduce sonar cut-off percentage to 60%.
Add desired sonar altitude to dataflash log's CTUN message.
Copter: AUTO ch7/ch8 option
Copter: fixed PX4 build
Copter: remove x100 from IMAX definitions