lib: rework the yang schema node iteration functions

lib: rework the yang schema node iteration functions

* Rename yang_snodes_iterate() to yang_snodes_iterate_subtree() and
  expose it in the public API.
* Rename yang_module_snodes_iterate() to yang_snodes_iterate_module().
* Rename yang_all_snodes_iterate() to yang_snodes_iterate_all().
* Make it possible to stop the iteration at any time by returning
  YANG_ITER_STOP in the iteration callbacks.
* Make the iteration callbacks accept only one user argument and not
  two.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>