Improve potion color alchemy in favor of players

 1. All random potion colors will map to an in-game potion.
 2. Full healing, gain level, gain ability are always secondary colors.

In testing I noticed characters were entering Gehennom with worse
enchantment on weapons/armor and had less HP to confront the tougher
UnNetHack Gehennom monsters and new resistance system, so this should
hopefully compensate for that somewhat with alchemy.