cgi: use transaction_complete when stdout goes away

transaction_complete includes processing for the case we are holding
an output-side buflist, and arranges for it to be sent before the
connection is actually closed.

This stops very slow links not being ablet to use git clone via
cgi mount, because the close races the flushing of the output-side
buflist and in the case the connection is bad, closes before
everything was sent.

lib/core/dummy-callback.c

@@ -155,7 +155,9 @@ lws_callback_http_dummy(struct lws *wsi, enum lws_callback_reasons reason,
 						   LWS_WRITE_HTTP_FINAL);
 
 			/* always close after sending it */
-			return -1;
+			if (lws_http_transaction_completed(wsi))
+				return -1;
+			return 0;
 		}
 #endif
 #if defined(LWS_WITH_HTTP_PROXY)
@@ -459,10 +461,12 @@ lws_callback_http_dummy(struct lws *wsi, enum lws_callback_reasons reason,
 			lwsl_debug("LWS_CALLBACK_CGI_TERMINATED: ending\n");
 			wsi->reason_bf |= LWS_CB_REASON_AUX_BF__CGI_CHUNK_END;
 			lws_callback_on_writable(wsi);
-			lws_set_timeout(wsi, PENDING_TIMEOUT_CGI, 3);
+			lws_set_timeout(wsi, PENDING_TIMEOUT_CGI, 10);
 			break;
 		}
-		return -1;
+		if (lws_http_transaction_completed(wsi))
+			return -1;
+		return 0;
 
 	case LWS_CALLBACK_CGI_STDIN_DATA:  /* POST body for stdin */
 		args = (struct lws_cgi_args *)in;