Export the {get,set}-node-flags functions.

They have been omitted in the Horde3D beta 5 API commit.
Thanks to rplacd for the report.
Enable *feature* based selection of terrain and sound extension bindings.

If Horde3d has been compiled with the sound or terrain extensions you
can enable the respective bindings by pushing
:horde3d-terrain-extension or :horde3d-terrain-extension to *features*
before loading the system.