Adds a new maintenance script refreshCategoryCounts.php
Following bug17155, the category population counts on WMF wikis aren't getting any *more* inaccurate, but they also aren't getting any more correct. A maintenance script needs to be run to refresh the cat_pages, cat_subcats and cat_files fields of the category table. I *think* that /maintenance/populateCategory.php would work, but that's not really what it was designed for, and it's certainly not the most efficient way of doing it. I've adapted it into a new script. So the "patch need-review" keywords are for someone to check that I haven't made some silly mistake on the patch, and commit it to svn. Then the "shell" is for someone to run it on all WMF wikis. If Simetrical did his job properly (:D) it should be easy on the servers.