FS#630: allow $conf['useheading'] to individually apply to content links and/or navigation links

$conf['useheading'] values are now:
- 0            : off, use page name in link text
- 'content'    : use first heading text for links in wiki page content
- 'navigation' : use first heading text for links in non-page content, e.g. breadcrumps, backlinks, search results, etc.
- 1            : use first heading text in all links
(for backwards compatibility, any other values are mapped to 0 or 1 by empty() function.)

$conf['useheading'] value should now be checked using the useHeading($linktype) function,
where linktype can be "content" or "navigation"

darcs-hash:20081119140758-f07c6-6e26456d50dcecc949fada31b0d4e72877fde1cc.gz