RSS/Atom: add a parameter to print only the N last links

It could be interesting to show only the N links when there is `?do=atom` or `?do=rss` in the URL, especialy for modules for WP, Joomla or personal ones. The GET parameter is `nb`.
If the request number is superior of the total count of links, it will display all links.

Few examples:  

    ?do=rss&nb=25
    ?do=rss&searchtags=security&nb=10