ipfw: create a bpf tap point for every log rule

[glebius]

Summary

Dynamically allocate bpf tap points for every rule that has "log".
The name is "ipfw%u", where %u is substituted to the rule number.
The default catch all "ipfw0" tap still exists for compatibility
and it will catch packets in case if there are no bpf listeners
on a per-rule tap.