UDP Encapsulation.

UDP Encapsulation.

A UDP-encap object that particiapates in the FIB graph and contributes
DPO to teh output chain. It thereofre resembles a tunnel but without the
interface. FIB paths (and henace routes) can then be created to egress
through the UDP-encap. Said routes can have MPLS labels, hence this also
allows MPLSoUPD.
Encap is uni-directional. For decap, one still registers with the UDP port
dispatcher.

Change-Id: I23bd345523b20789a1de1b02022ea1148ca50797
Signed-off-by: Neale Ranns <nranns@cisco.com>