Remove the `cmp` argument for the `avl_add()` function
Removed `cmp` argument for `avl_{del,find,unlink}`

This patch removes the `cmp` argument for the functions

    * avl_del
    * avl_find
    * avl_unlink

and replaces it with a `pred` (predicate) argument, as we somehow need a
selector for the functions.
Fixup for 9ce9efbb98a1f6d132598701324ca1b53f7bb7f8