Don't return a DHCPv6 reply in response to a confirm without address(es)

RFC3315 Section 18.2.2 states no reply must returned by the server in case no address(es) are present :
If the server is unable to perform this test (for example, the server does not have information about prefixes on the link to which the client is connected),
or there were no addresses in any of the IAs sent by the client, the server MUST NOT send a reply to the client.
Add status code ok option in reply to a decline message

RFC3315 Section 18.2.7 states a status code OK needs to be returned by the server :
After all the addresses have been processed, the server generates a  Reply message and
includes a Status Code option with the value Success, a Server Identifier option with
the server's DUID, and a Client Identifier option with the client's DUID.
Merge pull request #13 from dedeckeh/bugfixes

Bugfixes