Fixed healthy-percentage-reduction-during-malfunction bug.

Crew system malfunction is implemented by scaling down healthy
percentage, the original intent apparently being to reduce it by 50%,
25%, and 10% for each severe, medium, and low malfunction.  As
implemented, the healthy percentage was reduced to (not by) 50%, 25%,
and 10% of its value for each severe, medium, and low malfunction,
resulting in low malfunctions being more severe than a severe ones.

* Applied healthy percentage reduction correctly for severe, medium,
  and low malfunctions.

* Made healthyPercentage (which is not used outside the method) a local
  variable; parallels productionRate usage.

productionRate