network: Fix IPv6 DHCP logic
network: Fix IPv6 DHCP logic

 - Re-order the code so that DHCPv6 can actually be turned off.
 - Use the ::,constructor= syntax to properly instruct dnsmasq not to do
   DHCPv6 when not needed.

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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