ipfw3: support bpf filter in layer4 module

ipfw3: support bpf filter in layer4 module

syntax:
bpf "<bpf string>"
e.g.
ipfw3 add allow all bpf "icmp and src 8.8.8.8"

bpf can be used to filter the packet payload.

pcap_compile_nocap was used to compile the bpf string
and bpf_filter for the filtering.