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-client.c

@@ -589,3 +589,9 @@ wl_proxy_get_id(struct wl_proxy *proxy)
 {
 	return proxy->object.id;
 }
+
+WL_EXPORT void
+wl_log_set_handler_client(wl_log_func_t handler)
+{
+	wl_log_handler = handler;
+}