implement keyword 'unwho ALL' for clearing all "wmllint: who" entries at end of campaign

I realized that as it stood, my dictionary would linger, bad if wmllint were being run on multiple campaigns. A special unwho keyword, 'all', clears the dictionary.
"wmllint: unwho ALL" who entries at LoW's final battle
wmllint: bugfix "unresolved scenario" check
wmllint-1.4: revise help to reflect its current status as an adjunct to current wmllint