AP_TECS : Reduce tendency to overspeed during climbout

The old code relies on the action of the integrator to raise the nose above the minimum pitch angle which for higher powered models can be too slow to correct. This results in the airspeed at the top of the take-off climb being too far above the set speed, with the result that a height overshoot occurs. Biasing the pitch angle so that zero speed error produces a pitch demand at the take-off minimum (rather than zero pitch angle) reduces the amount of time before an over-speed condition is corrected.