Improved links to usenet messages on the web.

Improved links to usenet messages on the web.

Tassilo Horn writes:

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.