If I have input to ipseckey(1M) like this;
add esp spi 0x1111 src foo dst bar encralg des encrkey 1234567890abcdef \
nat_loc external-ip
ipseckey(1M) will return a PF_KEY error about an unknown extension. This is because
we allocate the space for the nat_loc address but do not fill it in.
We need to check for "encap udp", which turns on NAT-Traversal additions for an SA.