Fix some ineffective backslashes in string literals

Fix some ineffective backslashes in string literals

Deal with lone backslashes that have no effect in string literals,
but indicate that something is amiss.

* lisp/auth-source-pass.el (auth-source-pass-entries):
* lisp/textmodes/artist.el (artist-figlet-get-font-list-windows):
* lisp/org/ob-abc.el (org-babel-expand-body:abc, org-babel-execute:abc):
* lisp/org/ob-forth.el (org-babel-forth-session-execute):
* lisp/vc/vc-git.el (vc-git--program-version):
Add backslash in regexp for correctness.

* lisp/gnus/nnmail.el (nnmail-split-abbrev-alist):
Replace `\||' with `\\|' to follow the obvious regexp intent.

* lisp/org/org-list.el (org-plain-list-ordered-item-terminator):
Add backslash in doc comment so that it appears as intended.

* lisp/progmodes/cc-engine.el (c-forward-decl-or-cast-1, c-end-of-decl-1):
* lisp/progmodes/f90.el (f90-font-lock-keywords-2):
* lisp/progmodes/etags.el (etags-tags-completion-table):
* lisp/progmodes/ruby-mode.el (ruby-syntax-propertize):
* test/lisp/emacs-lisp/cl-print-tests.el (cl-print-tests-1):
Remove superfluous backslashes from regexp.

* test/lisp/emacs-lisp/rx-tests.el (rx-char-any):
Remove superfluous backslash from doc comment.