Fix Tramp error handling process buffer

* lisp/net/tramp-cache.el (tramp-list-connections):
* lisp/net/tramp-cmds.el (tramp-cleanup-connection): Use connection
property " connected" for a check of active connections.

* lisp/net/tramp.el (tramp-get-buffer): Set connection property
" connected".

lisp/net/tramp-cache.el

@@ -554,7 +554,7 @@ PROPERTIES is a list of file properties (strings)."
      (lambda (key)
        (and (tramp-file-name-p key)
 	    (null (tramp-file-name-localname key))
-	    (tramp-connection-property-p key " process-buffer")
+	    (tramp-connection-property-p key " connected")
 	    key))
      (hash-table-keys tramp-cache-data))))
 

lisp/net/tramp-cmds.el

@@ -173,7 +173,7 @@ interactively, a Tramp connection has to be selected."
 		     (get-buffer (tramp-debug-buffer-name vec)))
 		   (unless keep-debug
 		     (get-buffer (tramp-trace-buffer-name vec)))
-		   (tramp-get-connection-property vec " process-buffer")))
+		   (tramp-get-connection-property vec " connected")))
       (when (bufferp buf) (kill-buffer buf)))
 
     ;; Flush file cache.

lisp/net/tramp.el

@@ -2002,10 +2002,11 @@ Unless DONT-CREATE, the buffer is created when it doesn't exist yet."
   (or (get-buffer (tramp-buffer-name vec))
       (unless dont-create
 	(with-current-buffer (get-buffer-create (tramp-buffer-name vec))
-	  ;; We use the existence of connection property " process-buffer"
-	  ;; as indication, whether a connection is active.
+	  ;; We use the existence of connection property " connected"
+	  ;; as indication, whether a connection is active.  It keeps
+	  ;; the connection buffer, for cleanup.
 	  (tramp-set-connection-property
-	   vec " process-buffer"
+	   vec " connected"
 	   (tramp-get-connection-property vec " process-buffer"))
 	  (setq buffer-undo-list t
 		default-directory