vrouter changes to support ping and traceroute diagnostics

    - Packets with transport header checksum of 0xffff are marked as
    diagnostic packets and trapped to agent for further processing
    - vrouter now decrements ttl for every packet forwarded
    - for diagnostic packets, vrouter copies the ttl from the inner
    packet to outer header
    - if ttl goes to zero, packets are trapped to agent so that ICMP
    errors are sent to the source