Add ATF for IPv6 NAT-T.

We use IPv6 NAT-T to avoid IPsec slowing down caused by dropping ESP packets
by some Customer Premises Equipments (CPE). I implement ATF to test such
situation.

I think it c

Add ATF for IPv6 NAT-T.

We use IPv6 NAT-T to avoid IPsec slowing down caused by dropping ESP packets
by some Customer Premises Equipments (CPE). I implement ATF to test such
situation.

I think it can also work with nat66, but I have not tested to the fine details.

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Add test cases of NAT-T (transport mode)

A small C program is added to make a special socket (UDP_ENCAP_ESPINUDP)
and keep it to handle UDP-encapsulated ESP packets.