Use a list of tested Net::FTP versions

The default pkgsrc Perl is 5.20, which comes with Net::FTP 2.79. I've used Net::INET6Glue with it for a few weeks now without problems (apart from getting repeated email from cron due to the warning).

My first choice was "if none", but List::Util in Perl 5.16 has neither "none" nor "any" available. Using "unless first" allows me to share this code between pkgsrc and OS X.