samples/pktgen: Add sample scripts for pktgen facility

These are Robert Olsson's samples which used to be available from
<ftp://robur.slu.se/pub/Linux/net-development/pktgen-testing/examples/

>
but currently are not.

Change the documentation to refer to these consistently as 'sample
scripts', matching the directory name used here.

Cc: Robert Olsson <robert@herjulf.se>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>