Integrated Marco's generic/improved cache into Drupal.  Requires an
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.