as a three-speed supercharger. The first stage was always engaged. The second stage
could be disengaged ("neutral"), in "low" speed or in "high" speed.
* Checklists/climb.xml: adjust; add reference.
* f4u-set.xml: add a rating. Adjust keybindings:
Alt+Q: shift supercharger speed down.
Alt+S: shift supercharger speed up.
* f4u-yasim.xml: use "neutral" supercharger for approach.
