pcapdump: support BPF filtering offline capture files
pcapdump: support BPF filtering offline capture files

This can be convenient when working on larger captures without having to
cherry-pick the packets into another PCAP file.

rouzier/gopacket,google/gopacket,xuzhaokui/gopacket,levigross/gopacket,levigross/gopacket,google/gopacket,google/gopacket,rouzier/gopacket,levigross/gopacket,xuzhaokui/gopacket,rouzier/gopacket,xuzhaokui/gopacket