ppp: set IPv6LL address from IPV6CP IID when addr-gen-mode=eui64

Currently the interface identifier for constructing IPv6 link-local
and SLAAC addresses for PPP interfaces is the stable-privacy one.

According the RFC 5072, the interface identifier can also be set from
the IPv6 Control Protocol. We don't have a specific addr-gen-mode
option for that, but since EUI64 can't be used for PPP, let's override
"addr-gen-mode=eui64" to mean that the IID from PPP should be used.
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