Plane: added TKOFF_THR_MIN

[tridge]

this allows for a minimum throttle to be applied during a takeoff or quadplane forward transition in auto throttle modes. This is useful on high drag aircraft where normal TECS throttle control results in a slow transition

[Georacer]

I suspect that the reason this has been made in Servos.cpp is to take into account the transition case. But I still think that TECS should be notified about the override nonetheless; it will allow it to apply integrator antiwindup, if necessary.

[tridge]

@Georacer updated to handle SLT transition