smart cache-control headers for media fetch

The HTTP Cache-Control header output now respects the Dokuwiki media parameters
'cache', 'recache', and 'nocache'.
  - cache: cachetime or one hour (which ever is larger)
  - recache: remaining cachetime + 10 seconds so the newly recached media is used
  - nocache: no caching

Notes:
  - 'proxy-revalidate' should ensure that public caches always revalidate, this
    will keep caches from (accidentally) ignoring Dokuwiki's ACL rules.
  - 'no-transform' should keep media from being modified in transit by caches.

Impact:
Speeds up page layout/display by browsers when they already have the media
in their [memory] cache. For example the 2nd time a page is viewed. This
also reduces server load by eliminating most of the overhead needed to
return '304 Not Modified' responses each time an image is viewed.

darcs-hash:20060421025424-05dcb-23ff26c5cb410bcd166299a840f4c500fa0d112e.gz