The code had g.rc_3.control_in which would bypass the channel mapping and meant
arming could only occur if the throttle was always mapped to channel 3.
Updated to use the channel mapping object.

fixes issue #1720

this should produce better glide slopes when the aircraft is off
course

try to account for wind and current ground speed to dynamically update
glide slope in landing to flare at the right point

The sensor driver runs asynchronously at 10Hz, so needs to be checked frequently for arrival of data.

Fixes SITL compile errors

when a change altitude command comes in while a large glide slope
altitude change is present we could end up using the old glide slope
with the new altitude. This resets the altitude offset, causing a
direct altitude change

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

thanks to BretC for reporting