Fix advertised IPv6 addresses

Fix advertised IPv6 addresses

The dns_reply_a() function appears to filter out IPv6 addresses other
than link-local. This means that advertised services are unreachable
over IPv6 (since on the client side, we drop the interface identifier
so link-local addresses aren't usable anyway).

Fix it to include all addresses, in accordance with RFC6762 §6.2:

   When a Multicast DNS responder sends a Multicast DNS response message
   containing its own address records, it MUST include all addresses
   that are valid on the interface on which it is sending the message,
   and MUST NOT include addresses that are not valid on that interface
   (such as addresses that may be configured on the host's other
   interfaces).  For example, if an interface has both an IPv6 link-
   local and an IPv6 routable address, both should be included in the
   response message so that queriers receive both and can make their own
   choice about which to use.  This allows a querier that only has an
   IPv6 link-local address to connect to the link-local address, and a
   different querier that has an IPv6 routable address to connect to the
   IPv6 routable address instead.