samples: pktgen: new append mode

To configure various complex flows we for sure can create custom
pktgen init scripts, but sometimes thats not that easy.

New "-a" (append) option in all the existing sample scripts allows
to append more "devices" into pktgen threads.

The most straightforward usecases for that are:
- using multiple devices. We have to generate full linerate on
all physical functions (ports) of our multiport device.
- pushing multiple flows (with different packet options)

Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>