This was definitely not working (no caller to auto_trim()), and has probably subtly broken in several other ways since it has not been autotested.
If this feature is wanted in the future then factoring Copter's support up into RC_Channel (and possible AP_Vehicle) is probably the way to do this.
we're moving towards using an RC aux switch for this functionality.
Blimp doesn't currently have suport for this functionality (lacking a call
