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/lpr.el

@@ -49,7 +49,7 @@
 (defcustom printer-name
   (and (eq system-type 'ms-dos) "PRN")
   "The name of a local printer to which data is sent for printing.
-\(Note that PostScript files are sent to `ps-printer-name', which see.\)
+\(Note that PostScript files are sent to `ps-printer-name', which see.)
 
 On Unix-like systems, a string value should be a name understood by
 lpr's -P option; otherwise the value should be nil.