Plane: prevent baro reset from OVERRIDE_CHAN

using OVERRIDE_CHAN would cause a baro reset as it would mean we are
temporarily disarmed, and the "reset baro and GPS when disarmed" check
would reset the baro

Now we only do the mixer update if disarmed
AP_TECS: make flare height demand consistent with height rate demand
AP_TECS: Fix bug in parameter label
AP_TECS: Add parameter to adjust height rate gain during flare
AP_TECS: Change default landing damp gain to 0.5