Add and remove backslashes in regexps

Add and remove backslashes in regexps

These irregularities were found by relint; see
https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00949.html .

* doc/lispref/modes.texi (Example Major Modes):
* etc/srecode/el.srt:
* lisp/cedet/data-debug.el (data-debug-mode):
* lisp/cedet/semantic/grammar.el (semantic-grammar-mode):
* lisp/cedet/srecode/srt-mode.el (srecode-template-mode):
* lisp/comint.el (comint--unquote&requote-argument):
* lisp/emacs-lisp/lisp-mode.el (lisp-mode):
* lisp/gnus/mm-uu.el (mm-uu-type-alist):
* lisp/progmodes/cc-awk.el (c-awk-harmless-pattern-characters*):
* lisp/progmodes/cfengine.el (cfengine-common-settings):
* lisp/progmodes/cperl-mode.el (cperl-after-sub-regexp, cperl-init-faces):
* lisp/shell.el (shell-chdrive-regexp, shell--unquote&requote-argument):
* lisp/textmodes/tex-mode.el (tex-common-initialization):
Remove duplicated backslashes in character alternatives.

* lisp/emacs-lisp/lisp-mode.el (lisp-el-font-lock-keywords-2):
* lisp/progmodes/opascal.el (opascal--syntax-propertize):
* lisp/progmodes/pascal.el (pascal--syntax-propertize):
Remove backslashes escaping non-special characters.

* lisp/progmodes/fortran.el (fortran-font-lock-keywords-3): Escape '*'.
* lisp/progmodes/perl-mode.el (perl-syntax-propertize-function):
Escape '^'.