ngircd: Add support for SSL=optional

If SSL.Ports is set and Global.Ports is not set, Global.Ports defaults
to empty (not 6667), and you don't get a plaintext listener [1].  With
the new 'optional' setting, explicitly declare Global.Ports so you
have both an SSL listener (on 6697) and a plaintext listener (on 6667).

[1]: http://arthur.barton.de/pipermail/ngircd-ml/2011-February/000471.html