FreeBSD complains otherwise.

In --bind-dynamic mode, stop listening on an address when it's
removed from an interface. 6rd and 6to4 tunnels can go through
lots of addresses.

Previously, if the DUID wasn't read from the lease-file or
script, a new one was created _after_ the helper process fork,
so for that first run, the script calls got an empty DUID.

Also, use a DUID_LL format DUID when there's no stable lease
storage, as well as when the RTC is broken. That has a chance of
evaluating to the same value on each startup.