trafgen: Dump proto headers in *.cfg format

trafgen: Dump proto headers in *.cfg format

Added trafgen_dump.c module which dumps headers from packet
in .cfg format. Packet is dumped if -o <file>.cfg was specified,
it might be useful to specify *.pcap file as input and convert it
into .cfg file to edit proto fields in human readable format.

To make it possible several main changes were added:

    1) packet id is embedded into struct packet.id, and
       it is updated on each realloc_packet()

    2) Added new struct proto_hdr.get_next_proto callback
       to make possible apply fields of next header.

    3) Added new dev_io ops for writting packets into .cfg file,
       to re-use common dev_io mechsnism for packets dumping.

Before dump the default ETH_PROTO fields are applied as first header and
then next proto_hdr is identified via .get_next_proto(...) callback.

Meanwhile only eth, arp, vlan, ip4, udp, & tcp protos can be dissected
into *.cfg format.

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>