Merge branch 'pktgen-scripts-improvements'

Igor Russkikh says:

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pktgen: scripts improvements

Please consider small improvements to pktgen scripts we use in our environment.

Adding delay parameter through command line,
Adding new -a (append) parameter to make flex runs

v3: change us to ns in docs
v2: Review comments from Jesper

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