deletion bug fixed that could orphan a large section of the tree.  the
  "parent" argument to rotate_left or rotate_right was sometimes not
  really the parent of the rotation vertex, because the parent's parent
  was not correctly reidentified after a rotation done on the parent.

Minor:
  forward static declaration of dns_rbt_printnodename, useful in debugging.

  level_matches set correctly for exact match in dns_rbt_findnode (it was
  one too few, but this was minor because as yet level_matches is only
  used by other code when a partial match was made.

  insist 'node' is a child of 'parent' in rotate_left and rotate_right

  compare a pointer explicitly to NULL that was just "if (pointer)"

  "would would" in a comment changed to just one "would".