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.h

@@ -96,6 +96,8 @@ uint32_t
 wl_display_get_global(struct wl_display *display,
 		      const char *interface, uint32_t version);
 
+void wl_log_set_handler_client(wl_log_func_t handler);
+
 #ifdef  __cplusplus
 }
 #endif