Fix some side-effecting uses of make-text-button

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2020-06/msg00117.html

* lisp/apropos.el (apropos-library-button):
* lisp/help-fns.el (help-fns--first-release): Return result of
make-text-button instead of relying on its side effects.
* lisp/ibuf-ext.el (ibuffer-old-saved-filters-warning): Avoid
modifying an immutable string.

lisp/apropos.el

@@ -665,8 +665,7 @@ Return list of symbols and documentation found."
       (make-text-button name nil
                         'type 'apropos-library
                         'face 'apropos-symbol
-                        'apropos-symbol name)
-      name)))
+                        'apropos-symbol name))))
 
 ;;;###autoload
 (defun apropos-library (file)

lisp/help-fns.el

@@ -647,8 +647,7 @@ FILE is the file where FUNCTION was probably defined."
                     (setq place (list f pos))
                     (setq first version)))))))))
     (when first
-      (make-text-button first nil 'type 'help-news 'help-args place))
-    first))
+      (make-text-button first nil 'type 'help-news 'help-args place))))
 
 (add-hook 'help-fns-describe-function-functions
           #'help-fns--mention-first-release)

lisp/ibuf-ext.el

@@ -202,7 +202,7 @@ The format has been repaired and the variable modified accordingly.
 You can save the current value through the customize system by
 either clicking or hitting return "
           (make-text-button
-           "here" nil
+           (copy-sequence "here") nil
            'face '(:weight bold :inherit button)
            'mouse-face '(:weight normal :background "gray50" :inherit button)
            'follow-link t