Make Pcap#setfilter behave more like tcpdump's setfilter
Make Pcap#setfilter behave more like tcpdump's setfilter

When setting a BPF filter against an interface that doesn't have an
IPv4 address, pcap_lookupnet() returns an error like:

"WARNING: <iface>: no IPv4 address assigned"

the tcpdump source code (tcpdump.c:1230) outputs this as a warning then
continues, setting net and mask to 0. This behaviour lets you use
BPF (with a few caveats - behaviour is different in a few cases) on
interfaces lacking an IPv4 address.

pcaprub/pcaprub,pcaprub/pcaprub