samples: pktgen: fix proc_cmd command result check logic

Currently, proc_cmd is used to dispatch command to 'pg_ctrl', 'pg_thread',
'pg_set'. proc_cmd is designed to check command result with grep the
"Result:", but this might fail since this string is only shown in
'pg_thread' and 'pg_set'.

This commit fixes this logic by grep-ing the "Result:" string only when
the command is not for 'pg_ctrl'.

For clarity of an execution flow, 'errexit' flag has been set.

To cleanup pktgen on exit, trap has been added for EXIT signal.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>