nav_form() renders links instead of a form.

Renders prev and next links, 5 page numbers, and up to 4
jump links depending on the viewed page.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5297 2fea6a4d-a838-0410-917b-93a53c48e9d2
Clean left overs.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5298 2fea6a4d-a838-0410-917b-93a53c48e9d2