SQL update. See below for more details.
- Merged the file "cache.inc" into "common.inc".
- In addition, I renamed the field 'url' in the cache table to
a more generic 'cid' (cache identifier). It's no longer for
URLs only.
- Made the "cache_set()" function ASNI compliant such that it
will play nice with other databases such as Postgres.
- Added some extra input checking.
- Updated the old caching code in the functions "page_header()"
and "page_footer()" to use the new, generic cache API.
- Updated "update.php" to make the required SQL changes.