* Adds Category and CategoryList classes to represent categories themselves.
* Adds a category table, giving each category a name, ID, and counts of all members, subcats only, and files.
* Adds a maintenance script to populate the category table efficiently. This script is careful to wait for slaves and should be safe to run on a live database. The maintenance script's includes file is called by update.php.
* Until the category table is populated, the patch handles weird category table rows gracefully. It detects whether they're obviously impossible, and if so, it outputs appropriate messages.