StrafeAirMoveType.cpp minimum throttle control

[KyleAnthonyShepherd]

As implemented, a StrafeAirMoveType unit chasing and attacking another air unit can only reduce throttle to 30% of its maximum.
This leads to some fast aircraft unable to stay behind slower aircraft.
https://github.com/beyond-all-reason/spring/blob/BAR105/rts/Sim/MoveTypes/StrafeAirMoveType.cpp#L785

const float angleLim = 1.0f - goalDotFront * 0.7f;

I propose a new UnitDef that allows the 0.7f factor to be defined on a per-unit basis, so that units can disable throttle reduction (value of 0) and make full speed flybys, or even be able to apply reverse throttle (value greater than 1) if rangeLim is also small enough.