Make connection and thread lists read-only again

This behavious was lost after the refactorings of
`slime-with-popup-buffer'.

* slime.el (slime-popup-buffer-mode): A SLIME popup buffer is
always read-only.

(cherry picked from SLIME commit
c8b595b2)

Conflicts:
	ChangeLog

sly.el

@@ -769,7 +769,8 @@ MODE is the name of a major mode which will be enabled.
 
 (define-minor-mode sly-popup-buffer-mode
   "Mode for displaying read only stuff"
-  nil nil)
+  nil nil nil
+  (setq buffer-read-only t))
 
 (add-to-list 'minor-mode-alist
              `(sly-popup-buffer-mode