show_help: only parse specific trees for help files #13247
                
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Change --root CLI param to --roots in order to accept a specific list of root directories to traverse to look for help files (rather than a single top-level root that contains all the directories that we want to traverse -- but which also contains directories that we don't want to traverse). Also, just for the heckuvit, parameterize the list of subdirectories to skip (via --skip-dirs).
Using --roots prevents us from accidentally traversing into VPATH build directories, for example, and finding PMIx and/or PRTE help files.