Add support for error causes.

gtests/net/packetdrill/lexer.l

gtests/net/packetdrill/packet.c

gtests/net/packetdrill/packet_to_string_test.c

gtests/net/packetdrill/parser.y

gtests/net/packetdrill/run_packet.c

gtests/net/packetdrill/sctp.h

gtests/net/packetdrill/sctp_chunk_to_string.c

gtests/net/packetdrill/sctp_iterator.c

gtests/net/packetdrill/sctp_packet.c

gtests/net/packetdrill/sctp_packet.h

...t/packetdrill/tests/bsd/sctp/sctp_error_causes_active.pkt