Inconsistent damage color coding thresholds of vehicle parts, fixed.

Problem can be seen when color of most damaged part is different from color of same part in parts list.
Always thought of the inconsistent damage colors as a bug, so I made this.
Uses the same damage color-codes for everything "vehicle" interaction related.
Since there were two sets of different damage color-codes, I wasn't entirely sure which one to use.
We may want to tweak or add some more color thresholds later, it can now be done in one place.
See getDurabilityColor in veh_interact.cpp for current values (thresholds unchanged).