First cut at making debug mode run time selectable

gtests/net/packetdrill/checksum_test.c

gtests/net/packetdrill/config.c

gtests/net/packetdrill/logging.h

gtests/net/packetdrill/packet_parser.c

gtests/net/packetdrill/packet_parser_test.c

gtests/net/packetdrill/packet_socket_pcap.c

gtests/net/packetdrill/packet_to_string_test.c

gtests/net/packetdrill/packetdrill.c

gtests/net/packetdrill/run_packet.c

gtests/net/packetdrill/run_system_call.c