Wayland: logging

The core libwayland libraries should not handle logging, only passing
the error messages to subscribed functions.
An application linked to libwayland-server or libwayland-client
will be able to set own functions (one per library) to handle error
messages.

Change in this series: make the wl_log return int, because
of compatibility with printf. It will return the number of bytes logged.

src/wayland-server.c

@@ -1178,3 +1178,9 @@ wl_client_new_object(struct wl_client *client,
 				    interface, implementation, id, data);
 
 }
+
+WL_EXPORT void
+wl_log_set_handler_server(wl_log_func_t handler)
+{
+	wl_log_handler = handler;
+}