Use standard example addresses in configs

`2001:db8::/32` is the [standard][1] IPv6 example address range, using
it also makes it clear when the user has just copy-pasted the example
config and didn't actually whitelist 6BONE for some strange reason.

Similarly, there are example addresses for [IPv4][2] and [DNS][3].

[1]: http://tools.ietf.org/html/rfc3849
[2]: http://tools.ietf.org/html/rfc5737
[3]: http://tools.ietf.org/html/rfc2606