lib: better support for nested YANG augmentations

lib: better support for nested YANG augmentations

Change the way the YANG schema node iteration functions work so that
the northbound layer won't have issues with more complex YANG modules
that contain multiple levels of YANG augmentations or modules that
augment themselves indirectly (by augmenting groupings).

Summary of the changes:
* Change the yang_snodes_iterate_subtree() function to always follow
  augmentations and add an optional "module" parameter to narrow down
  the iteration to nodes of a single module (which is necessary in
  some cases). Also, remove the YANG_ITER_ALLOW_AUGMENTATIONS flag
  as it's no longer necessary.
* Change yang_snodes_iterate_all() to do a DFS iteration on the resolved
  YANG data hierarchy instead of iterating over each module and their
  augmentations sequentially.

Reported-by: Rafael Zalamena <rzalamena@opensourcerouting.org>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>