Replace I-D.draft-byrne-v6ops-clatip with RFC 7335

I-D.draft-byrne-v6ops-clatip has been published as RFC 7335, update
documentation and comments accordingly.
Generate random IIDs if no EUI-64 address is found

This allows clatd to work correctly on 3GPP mobile networks, where the
IID is assigned from the network, rather than being generated using
EUI-64. We still prefer the old method, though, the random one is only
used if no EUI-64 address exists on the PLAT device. Update docs
accordingly.

Also upgrade docs to better describe usage as a SIIT-DC Host Agent.
Avoid a restart loop with NM dispatcher scripts

Newer NetworkManager versions will run the dispatcher scripts when new
unknown interfaces show up, including 'clat'. That will cause a clatd
restart right after startup, and we'll never get to fully initialise. So
ensure the dispatcher script does nothing if the device in question is
our own 'clat' to prevent this.