pktgen: add sample script pktgen_sample04_many_flows.sh

Adding a pktgen sample script that demonstrates how to use pktgen
for simulating flows.  Script will generate a certain number of
concurrent flows ($FLOWS) and each flow will contain $FLOWLEN
packets, which will be send back-to-back, before switching to a
new flow, due to flag FLOW_SEQ.

This script obsoletes the old sample script 'pktgen.conf-1-1-flows',
which is removed.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>