Make custom-data saving atomic

* lisp/cus-edit.el (custom-save-all): Do the custom saves as an
atomic change group, to avoid writing invalid data if something
goes wrong (or the user hits `C-g') in the middle (bug#18633).

lisp/cus-edit.el

@@ -4717,8 +4717,9 @@ if only the first line of the docstring is shown."))
       (let ((inhibit-read-only t)
 	    (print-length nil)
 	    (print-level nil))
-	(custom-save-variables)
-	(custom-save-faces))
+        (atomic-change-group
+	  (custom-save-variables)
+	  (custom-save-faces)))
       (let ((file-precious-flag t))
 	(save-buffer))
       (if old-buffer