Backslash cleanup in Elisp source files

Backslash cleanup in Elisp source files

This patch should not change behavior.  It typically omits backslashes
where they are redundant (e.g., in the string literal "^\$").
In a few places, insert backslashes where they make regular expressions
clearer: e.g., replace "^\*" (equivalent to "^*") with "^\\*", which
has the same effect as a regular expression.
Also, use ‘\ %’ instead of ‘\%’ when avoiding confusion with SCCS IDs,
and similarly use ‘\ $’ instead of ‘\$’ when avoiding confusion
with RCS IDs, as that makes it clearer that the backslash is intended.
* auth-source.el (auth-source-secrets-search)
(auth-source-macos-keychain-search):
* gnus-art.el (gnus-mime-action-on-part)
(gnus-mime-display-multipart-related-as-mixed)
(gnus-button-mid-or-mail-heuristic-alist)
(gnus-button-mid-or-mail-heuristic, gnus-article-extend-url-button):
* gnus-bookmark.el (gnus-bookmark-bmenu-toggle-infos)
(gnus-bookmark-bookmark-inline-details, gnus-bookmark-bookmark-details)
(gnus-bookmark-alist, gnus-bookmark-name-from-full-record)
(gnus-bookmark-bmenu-mode, gnus-bookmark-delete):
* gnus-cus.el (gnus-group-customize):
* gnus-group.el (gnus-group-line-format, gnus-group-mode-line-format)
(gnus-group-jump-to-group-prompt, gnus-group-name-at-point):
* gnus-mlspl.el (gnus-group-split-fancy):
* gnus-registry.el (gnus-registry-prune-factor):
* gnus-salt.el (gnus-summary-pick-line-format):
* gnus-sieve.el (gnus-sieve-update, gnus-sieve-generate)
(gnus-sieve-script):
* gnus-util.el (gnus-directory-sep-char-regexp):
* gnus.el (gnus-message-archive-group, gnus-user-agent):
* message.el (message-archive-header, message-reply-headers)
(message-send-method-alist):
* mm-decode.el (mm-add-meta-html-tag):
* nndoc.el (nndoc-generate-lanl-gov-head):
* nnheader.el (nnheader-max-head-length):
* registry.el (registry-db, registry-prune):
* rfc1843.el (rfc1843-hzp-word-regexp)
(rfc1843-hzp-word-regexp-strictly):
* rfc2047.el (rfc2047-encoded-word-regexp)
(rfc2047-encoded-word-regexp-loose):