* lisp/emacs-lisp/re-builder.el: Cosmetic changes

* lisp/emacs-lisp/re-builder.el: Cosmetic changes

(reb-valid-string): Declare it risky so the mode-line will obey its
text-properties.
(reb-lisp-syntax-p): No need to make it `defsubst`.
(reb-target-value): New function to replace the
`reb-target-binding` macro.  Adjust all callers.
(reb-auto-update): Give a more informative error message than just
"invalid" and give it a warning face.
(reb-update-regexp): Always update `reb-regexp-src` (it's harmless),
rather than only when it's necessary (reduces the need for advice in pcre.el).