Consolidate duplicated ghostly sentinel code.
Raise ilist_node::(get|set)(PrevNext) to protected.
This allows subclasses to use "using" clauses to disambiguate the ilist_nextprev_traits accesses if they have inherited other methods of the same name from elsewhere.
Clean-up andersen ilist code.
Now using the new ilist_ghostly_sentinel_traits and protected (get|set)(Prev|Next) methods.