more precise range conversion for SBus input signals

Improved range conversion of Futaba SBus signals. -+100% on TX now result in values between 1000 and 2000µs. This range is still deviating from value got by PPM input, but the PX4 seems to expect exactly this range internally, e.g. for applying deadzones. That should not be a problem, as all limits hardcoded in APM (e.g. 1850ms) are easily reachable (in contrast to the original implemtation). 

Some defines allow to easily adjust the target range. Added some comments about SBus ranges.