pktgen: Add example script on howto use these pktgen functions

I'm just going to "release" this example script, even-though
I would have liked to fixed the ugly pktgen setting of global
variable PGDEV.

I will not promise to keep the variable PGDEV setting API stable,
but to be realistic I don't have time to fix/cleanup this stuff.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>