Applied patch by Natrak:

- page_header() now adds Last-Modified and ETag http headers.
- When running PHP as an Apache module page_header() will check the HTTP
  headers for conditional gets, and will only push the content when it
  fails. (Works for html and xml pages as they are all cached). Note:
  this is a PHP limitation, so until PHP makes it work for other web
  servers this won't work for them.
- Added created field to cache database to hold the timestamp when the
  cache was created.
- Changed cache_get() to return an object with ->data and ->created.
- Update forum and locale modules.