More RFC-conformable to NDP/SAA

Inforced message handling for incoming NDP/SAA packet
using a unified management function
discard illegal NDP messages BEFORE process
Refined neighbour tables
IPv6 needs much more special / strict state transition than others.
We've also refined locking mechanisms.
Privacy Extentions (RFC3041)
periodically-generated "temporary addresses" in addition to "public address"