1588-2008 support in GS724Tv4 firmware

The 802.1AS firmware for the GS724T switch already handles almost everything needed for full 1588 support, it would be useful to add the capability to use layer 3 transport for the 1588 timestamps so that the GS switch could work as a 1588 transparent clock in IP based systems, not just layer 2 802.1AS transport.