Add zen_sort_array() and optimize debug-log widget

Add zen_sort_array() multisort function and apply to debug-log-list widget
- removed duplicate building of logfile array to save memory (used in two places: widget and init-system)
- apply maximum limit to list in widget
- add summary line to log widget

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/zencart/zc-v1-series/227)
<!-- Reviewable:end -->