The nudge value depended on RC3_TRIM, which is not a reliable
value. It tried to only take effect when the throttle stick was above
50%, but if RC3_TRIM was high for some reason (say 1500) then it would
actually depress the throttle by a large amount, which could cause the
plane to stall.

This also adds a boolean option THROTTLE_NUDGE to allow disabling of
throttle nudging