Icecast should expose the YP listing status to the public status pages

There are hundreds if not thousands of streams that fail to list properly in dir.xiph.org and it's obvious that nobody notices.

/admin/stats should expose a per mount status field if the given mount is set public and directory listings are enabled for the server.

Logic could be as follows:

• OK - No known errors for this mount
• WARN - During the last n yp-touches at least one error was received
• FAIL - No successful listing/touch for this mount

Further information, including verbatim messages from YP should be available through the admin interface. Covered in a separate ticket.