bugfix fetch remote media (recache and nocache)

Fixes a major bug in fetching remote media with 'recache' and improves
the efficiency of 'nocache'.

Recache:
  - Used to reload the remote media on EVERY request.
  - Now it behaves as intended and only reloads the remote media
    into the Dokuwiki cache every $conf['cachetime'] time.

Nocache:
  - No longer stores remote media in the Dokuwiki cache.
  - No longer loads, saves, and forwards remote media -- just redirects.
  - No longer resizes images on the server because the cached results
    can not be reused.
Overall this is faster for Dokuwiki. The bandwidth usage for
the 3rd party server is the same (less for Dokuwiki). Page loading
should also be faster because data is not being forwarded through
Dokuwiki (and the 3rd part server's cache control headers will be
respected automatically).

darcs-hash:20060421012210-05dcb-a6029baa0fad218ace28e0e3c2f442b1ca645a99.gz