[WIP] openbsd: stats from pf

[yjh0502]

While trying to monitor traffic on OpenBSD router box with darkstat, I observed that darkstat cannot account traffic accurately when there's a large traffic, because of packet pcap packet drop. For example, openbsd pcap drops about 1/3 packets on ~1gbps which makes accounting rather inaccurate.
To overcome the issue, I tried to capture number of bytes/packets from pf instead of pcap. It seems working on my environment and I wanted share the patch with others :)

Notes

• cache.c and pf.c are largely borrowed from openbsd source tree.
• pf should be enabled to make it work.
• It requires to set -l flag to work property, as it cannot detect local interface address.
• It does not handle ipv6 states.