ip6: don't 'assume' IPv6 connections (bgo #676740)
The bug report describes a testing scenario where device is down
and no IPv4 or IPv6 addresses are set up. Then NetworkManager
is started and it decides not to configure DHCPv6 address. This
is caused by code copied from IPv4 that does not work here at
all (and I wonder if it does in IPv4).
In any case, when NetworkManager starts a DHCP client and recieves
an IPv6 address (no addresses are configured at the time), it
*must* send this new IPv6 address to the kernel.