Add a "read" function pointer to the pcap_t structure, which handles

Add a "read" function pointer to the pcap_t structure, which handles
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.