Add _g_io_module_get_default(), use to simplify other *_get_default()s

Add _g_io_module_get_default(), which implements the
figure-out-the-best-available-module-that-is-actually-usable logic,
and use that to simplify g_proxy_resolver_get_default(),
g_settings_backend_get_default(), g_tls_backend_get_default(), and
g_vfs_get_default().

https://bugzilla.gnome.org/show_bug.cgi?id=620932

gio/giomodule-priv.h

@@ -30,6 +30,11 @@ G_BEGIN_DECLS
 void _g_io_modules_ensure_extension_points_registered (void);
 void _g_io_modules_ensure_loaded                      (void);
 
+typedef gboolean (*GIOModuleVerifyFunc) (gpointer);
+gpointer _g_io_module_get_default (const gchar         *extension_point,
+				   const gchar         *envvar,
+				   GIOModuleVerifyFunc  verify_func);
+
 G_END_DECLS
 
 #endif /* __G_IO_MODULE_PRIV_H__ */