Add softflowd, flow-based network traffic analyser capable of Cisco NetFlow
data export. Softflowd semi-statefully tracks traffic flows recorded by
listening on a network interface or by reading a packet capture file.
These flows may be reported via NetFlow to a collecting host or summarised
within softflowd itself.

PR: ports/73723
Submitted by: Janos Mohacsi <janos.mohacsi@bsd.hu>