reading packets from a pcap_t, and make "pcap_read()" call it. That
removes the last place where we have to check for a pcap_t that refers
to a DAG card rather than a live capture, so get rid of the "is_dag" flag.
getting statistics for a pcap_t. Have "pcap_stats()" call it, rather
than being a per-platform function; have stats routines for non-live
pcap_t's that return an error.