pflow: import from OpenBSD

pflow is a pseudo device to export flow accounting data over UDP.
It's compatible with netflow version 5 and IPFIX (10).

The data is extracted from the pf state table. States are exported once
they are removed.

Reviewed by: melifaro
Obtained from: OpenBSD
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D43106