taxonomy_term_count_nodes returns wrong count for duplicate children, may not cache right either

This issue was also raised against Drupal 6 in taxonomy_term_count_nodes returns wrong count.

I include this here as a separate issue because my patch is much simpler and dodes not change the API, and something along these lines may be preferable to the patches being considered there for Drupal 5 (maybe even Drupal 6).

I'll be following both issue queues and will try to update this patch with relevant suggestions.

See Agaric's notes on quick-fixing this function.

^{benjamin, Agaric Design Collective}