Add ability to send empty access ports

[marinmo]

Hello,

When using fwknopd there's an option (OPEN_PORTS) in access.conf that will open the specified ports if none are requested alongside the correct knock-sequence. However, there doesn't seem to be any way of doing this in fwknop2 as selecting "Open port" requires a port to be filled in. Could this option be added? This would increase security as sending a correct knock-sequence would not include which ports said sequence serves to open.

Thaks in advance