Changes

Hoverslam: add RF-compatible pulse width modulation

This integrates the requested acceleration in the vertical descent
and applies a PWM to the throttle with a configurable minimum
pulse width.

It doesn't know about limited ignitions and it will shift to PWM
below minThrottle on any engine and start to waste ignitions.
(but you really shouldn't have limited ignition engines that you
need lower than your minThrottle to land -- either deep throttle and
keep your authority somewhere in the middle of the band, or have
lots or infinite ignitions).
 (commit: f6f9e95)