Fit example conf file typo.
Improve RFC-compliance when unable to supply addresses in DHCPv6

While testing https://github.com/sbyx/odhcp6c client I have noticed it
permanently crashes after startup.

The reason was it (odhcp6c) doesn't expect empty IA options in ADVERTISE
message without any suboptions.

Despite this validation bug of odhcp6c, dnsmasq should not generate
ADVERTISE messages with IA if there's nothing to advert per  RFC 3315
17.2.2:

   If the server will not assign any addresses to any IAs in a

   subsequent Request from the client, the server MUST send an Advertise

   message to the client that includes only a Status Code option with

   code NoAddrsAvail and a status message for the user, a Server

   Identifier option with the server's DUID, and a Client Identifier

   option with the client's DUID.

Meanwhile it's need to add status code for every IA in REPLY message per
RFC3315 18.2.1:

   If the server cannot assign any addresses to an IA in the message
   from the client, the server MUST include the IA in the Reply message
   with no addresses in the IA and a Status Code option in the IA
   containing status code NoAddrsAvail.

So, I've changed the logic to skip IA completely from ADVERTISE messages and
to add NoAddrsAvail subcode into IA of REPLY messages.

As for overhead, yes, I believe it's ok to return NoAddrsAvail twice in IA
and in global section for compatibility with all old and new clients.
add chinadns feature