Copter: CH6 tuning definition clean-up

Renamed and reorganised the CH6 #defines and parameter definitions
Copter: rename g.pid_throttle to g.pid_throttle_rate

Small code clean-up to make throttle rate controller's pid more obvious
Rover: update version for new release
Rover: added LEARN_CH parameter

this allows the learning channel to be set, for transmitters where
setting up channel 7 is difficult
Rover: changed to 2.43beta1