Hi Carsten and Org-crew,

here's the promised refactoring of org-gnus.el:

 - Rename org-usenet-links-prefer-google to org-gnus-prefer-web-links
 - Make that option work for gmane
 - Only make weblinks if the article is in a newsgroup
 - Only make weblinks if the article has no X-No-Archive header

There is little drawback:

 - Gnus stored some headers in an array and makes them instantly
   available.  Unfortunately that doesn't apply to X-No-Archive, so I
   have to select and widen the article buffer and parse anew.

   But I guess that's not a real problem...

So here's the patch: