ancestor/level history no longer kept in rbt structure; node_chain structure
was created for this purpose, and each dns_rbt_deletename call will pass
the chain structure to dns_rbt_findnode.

color enum removed from rbtnode structure; single bit used instead

length of name in rbtnode no longer kept as int in rbtnode structure;
stored as single byte just past end of rbtnode, and the actual name data starts
two bytes past the end.

dns_rbt_create takes additional argument that is a pointer to a function
which is responsible for deleting the space pointed to by an rbtnode's data
pointer when the node is removed.

dns_rbt_addnode renamed to dns_rbt_addonlevel so that dns_rbt_addnode could
be a public function that can provide the address of the new node back
to the caller.

removed several #if 0 blocks that were held over from when parent pointers
were used.