transfer: Replace __func__ instances with function name

.. also make __func__ replacement in multi.

Prior to this change debug builds would fail to build if the compiler
was building pre-c99 and didn't support __func__.

lib/multi.c

@@ -1509,7 +1509,8 @@ static CURLMcode multi_runsingle(struct Curl_multi *multi,
         break;
       }
 
-      DEBUGF(infof(data, "%s:%d call Curl_readwrite\n", __func__, __LINE__));
+      DEBUGF(infof(data, "multi_runsingle:%d call Curl_readwrite\n",
+                   __LINE__));
 
       /* read/write data if it is ready to do so */
       result = Curl_readwrite(data->easy_conn, &done);