Improve nofollow behaviour for ?do=index

In response to FS#2766
- make namespace links in the browser sitemap nofollow
- remove nofollow from browser sitemap link on the wiki start page
  when sitemap.xml generation is disabled
include namespace links when index access from the wiki start page
and no automatic sitemap.xml is being generated.
simplify comment
Merge pull request #247 from splitbrain/FS#2766

Improve nofollow behaviour for ?do=index