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.