The URL of the page is now treated differently related to different page
generation codes and at import:
- it is possible to have duplicates of a URL fragment in the database if
the full URLs are different
- if a secnd URL fragment is trying to be inserted at the import time,
providing an already existant full URL, then the URL fragment id renamed
by the concatenation of _1, _2 etc.
- in the page generation codes, the page links are determined relative
to the current page.