. Fix sense of rb_tree_iterate(rbt, NULL, ...).
. Document RB_TREE_MIN/RB_TREE_MAX to avoid relying on that sense.
. Document RB_TREE_FOREACH and RB_TREE_FOREACH_REVERSE to simplify
iteration.
Addresses PR lib/46034 and PR lib/47144. It would have been nice to
make `x = NULL; while ((x = rb_tree_iterate(t, NULL, ...)) != NULL)'
DTRT to traverse t, but it's too much late for that now.
We probably ought to have an RB_TREE_FOREACH{,_REVERSE}_SAFE too.
ok christos
b757dad44d