Fix IPv6 linklocal addresses to DTRT.

If an interface isn't configured for an IPv6 address
and it's not configured to obtain an address via autoconfig
it shouldn't have an IPv6 linklocal address.

Before this commit FreeNAS always had a linklocal address
on any interface that was up.  This caused problems as services
like mDNSResponder would bind to the IPv6 address, then complain
when it couldn't send traffic via IPv6.

Ticket: #3965