[PKTGEN]: DSCP support

Anyway, I've been asked to add support for managing DSCP codepoints,
so one can test DiffServ capable routers. It's very simple code and is
working for me.

Signed-off-by: Francesco Fondelli <francesco.fondelli@gmail.com>
Signed-off-by: Robert Olsson <robert.olsson@its.uu.se>
Signed-off-by: David S. Miller <davem@davemloft.net>