[PATCH] mDNS support

The last outstanding reason to retain dnsserver component in Squid seems
to be lack of mDNS support in our client resolver. This patch seeks to
close that hole and add mDNS support to the Squid internal DNS resolver.

* adds the mDNS multicast group IPs as always-present entries in the
nameservers list.

* filters each request. ".local" lookups are permitted to both the mDNS
resolvers and the recursive resolvers, other requests are only permitted
to the regular recursive resolvers.

I have not done much testing of this yet. However the DNS protocol
operations are completely unchanged, and the only questions are whether
the nameservers responses are accepted back into Squid or prevented by
the ignore_unknown_nameservers option (or not) - that depends on the
mDNS responses. And whether the mDNS produces NXDOMAIN in a useful way
for Squid.

Amos