Add glib__private__() API to share between glib,gio; port GWakeup to it

Historically we've added random symbols to the public API with warnings
that they're private; examples are:

glib_gettext(), glib_pgettext()
g_thread_functions_for_glib_use, g_thread_use_default_impl, etc.

And we almost added "GWakeup" to public API just to share between glib and
gio.

This new glib__private__() API exports a hidden vtable, and adds a macro
GLIB_PRIVATE_CALL() that makes it generally convenient to use.

This adds an extremely tiny cost for the double indirection; but it has
the benefit that we don't need to either:

1) compile the code into both glib and gio (like GWakeup), with the
   inefficiency that implies.
2) Export a "do not use this" symbol; the serious problem with this is
   that someone CAN use it pretty easily.  Particularly if we document
   it.  It's far, far harder to peek into a structure without a public
   header file.

https://bugzilla.gnome.org/show_bug.cgi?id=657992

glib/glib-private.c

@@ -0,0 +1,45 @@
+/* GLIB - Library of useful routines for C programming
+ * Copyright (C) 2011 Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Author: Colin Walters <walters@verbum.org>
+ */
+
+#include "gutils.h"
+#include "glib-private.h"
+
+/**
+ * glib__private__:
+ * @arg: Do not use this argument
+ *
+ * Do not call this function; it is used to share private
+ * API between glib, gobject, and gio.
+ */
+GLibPrivateVTable *
+glib__private__ (void)
+{
+  static GLibPrivateVTable table = {
+    g_wakeup_new,
+    g_wakeup_free,
+    g_wakeup_get_pollfd,
+    g_wakeup_signal,
+    g_wakeup_acknowledge
+  };
+ 
+  return &table;
+}
+