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
This extension point allow extending GLib with library like LibProxy that
interprets system proxy settings and finds the appropriate configuration
based on the type of connection being made.
Reviewed-by: Dan Winship <danw@gnome.org>
