bin/list_lists needs an option for listing only lists with public archives.

[Mark Sapiro]

For example, if one wants to create a list of pages for public search engines to index, one can make a list of all the *.html files in the archives/private/<listname>/ directory, but one probably only wants to do this for lists with public archives. With an option for listing only lists with public archives, 'bin/list_lists --bare --public_archive' can easily create a list of those lists.