It is occasionally necessary to advertise multiple prefixes on the same
interface, particularly when those prefixes are not on-link. The *radvd*
role thus now expects each item in `radvd_interfaces` list to have a
`prefixes` property, which itself is a list of prefixes to advertise.
Prefixes can specify properties such as `on_link`, `autonomous`,
`preferred_lifetime`, etc.
Dustin
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