sync with more-recent LBL 0.4, about loopback interface detection
(/^lo[0-9]?$/).
CAVEAT: with GENERIC kernel on laptops laptops, pcap_lookupdev would almost
always pick eon0 as the interface, and fails because eon0 has no bpf
attachment. we may want to change pcap_lookup{,dev} to check if the
interface has bpf attachment or not.
almost in sync with tcpdump.org source code tree.