Write an ipv6 example (test LWIP_IPV6)

[peterharperuk]

We should build with LWIP_IPV6=1 as it will force us to treat addresses properly.
Some of the tests won't work with an IPV6 address but at least the code will be correct?
Note: There's a LWIP bug which stops ping working lwip-tcpip/lwip#17