if_bridge(4): Complete bpf auditing of local traffic over the bridge

[kevans]

Summary

There were two remaining "gaps" in auditing local bridge traffic with bpf(4):

Locally originated outbound traffic from a member interface is invisible to the bridge's bpf(4) interface. Inbound traffic locally destined to a member interface is invisible to the member's bpf(4) interface.

I call these "gaps" because they don't affect conventional bridge setups. Alas, being able to establish an audit trail of all locally destined traffic for setups that can function like this is surely not a bad thing.