Include PGP and GnuPG in Keywords, and other commentary refinements.

Include PGP and GnuPG in Keywords, and other commentary refinements.
(allout-abbreviate-flattened-numbering): Rename to
allout-flattened-numbering-abbreviation, and define-obsolete-variable-alias
the old name.
(allout-flattened-numbering-abbreviation): Rename from
allout-abbreviate-flattened-numbering.
(allout-mode-p): Include among autoloads, for use by other modes with impunity.
(allout-listify-exposed): Use allout-flattened-numbering-abbreviation.
(allout-encrypt-string): Use set-buffer-multibyte directly.
(allout-set-buffer-multibyte): Remove.