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.