rbtree.c: don't call delete_fn in rbtree_insert, return a value on duplicate keys instead.

for design reasons, it's more flexible to not use callbacks for this purpose.

docs/technical/rbtree.txt

@@ -39,8 +39,7 @@ Functions
 `rbtree_insert()`::
 
     Creates and inserts a new node in the tree. +
-    If provided, calls `rbtree->delete_fn` if a node should be updated. +
-    Returns nonzero if a new node was inserted or updated, zero otherwise.
+    Returns a positive value if the node was inserted, zero if the key already exists in the tree and a negative value if an error occured.
 
 NOTE: The memory pointed to by the 'key' pointer is *not* copied so you must ensure that it has a infinite lifetime.