- create_node() in rbt.c cannot fail.
- the dns_rbt_*name() functions, which are wrappers around
dns_rbt_[add|find|delete]node(), were never used except in tests.
- dns_rbnodechain_nextflat() and _down() were never used at all.
this change isn't really necessary since RBT is likely to
go away eventually anyway. but keeping the API as simple
as possible while it persists is a good thing, and may
reduce confusion while QPDB is being developed from RBTDB code.
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