[getdns-api] link-local recursive name server addresses

[Tony Finch]

Thomas Schäfer reported an interesting bug on the ipv6-ops list:
http://lists.cluenet.de/pipermail/ipv6-ops/2014-July/010032.html

The problem occurs when /etc/resolv.conf contains a link-local nameserver
address, which necessarily includes a scope so that the address is
associated with the correct interface.

Some stub resolver libraries fail to parse the scope - usually they ignore
the scope rather than failing, but this results in the wrong interface
index in the eventual sockaddr, so the resolver ends up unable to talk to
its server.

The interestingly awkward thing about this bug is that it implies that you
cannot use a simple IPv6 address (e.g. AAAA RDATA) to represent a stub
resolver's name server addresses. Unfortunately the getdns API assumes
that you can; to fix this it needs to learn about scoped addresses.

Tony.
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