New netstat function to determine the next available port

Based on draft commit
http://github.com/pannal/torrentflux/commit/d058771400b38fd86cb5f7e32703389a679ab028

To use the ipv6 line (:::######) instead of the ipv4 line (0.0.0.0:######) since newer
versions of Linux only listen on ipv6 but allow connections on ipv4 via ipv4 to ipv6 mapping.
So if you have a torrent and no one is leeching from you then pannal's version will have the
issue where two torrents try to run on the same port.

Ideally there would be a nice combination of ipv4 and ipv6 with the duplicates removed.

Also add basic support for windows netstat from win32gnu tools and macos x one too