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.

lisp/mail/mailclient.el

@@ -95,7 +95,7 @@ supported.  Defaults to non-nil on Windows, nil otherwise."
 		       recp)))
 	       (setq first nil))
 	     (split-string
-	      (mail-strip-quoted-names field) "\, *"))
+	      (mail-strip-quoted-names field) ", *"))
 	    result)))))
 
 (declare-function clipboard-kill-ring-save "menu-bar.el"