Radv proto sending adverts on wrong interface

[Juliusz Chroboczek]

> The field sin6_scope_id should be used only for link-local addresses (to
> define their scope), not as a way to route multicasts.
> 
> (Hmm, ff02::/16 is defined as link-local multicast address, so perhaps
> setting sin6_scope_id makes sense.)

FWIW, babeld uses the sin6_scope_id when sending its multicast packets (it
does not do the setsockopt), and we've never received a report about
packets going out the wrong interface.

-- Juliusz