Plane: move delay callback setup to after compass and airspeed config

this ensure parameters sent on startup include compass settings and
airspeed offset
Plane: fixed number of channels we pass to PWM limits
Plane: fixed potential division by zero in mixing creation

also setup failsafe values if already armed, so while loading new
mixer the throttle output doesn't go to zero
Plane: update OVERRIDE_CHAN docs
Plane: change default autotune level to 6
AP_HAL_AVR_SITL: fill in ublox accuracy metrics with reasonable values