Infinite recursion in get_category_parents() and _children()

[tellyworth]

These two functions can get stuck in a recursion loop if there's an indirect category hierarchy loop like A -> B -> C -> A.

The patch keeps track of traversed IDs and prevents repetition - there's a name for the algorithm but it escapes me. It increases memory usage but probably not too much - certainly less than infinite recursion does.

(In [8299]) Prevent category looping. Props tellyworth. fixes #7267

This should be able to leverage wp_check_term_hierarchy_for_loops() now.