trafgen: add option to not adjust system socket mem during testrun

The default behavior of trafgen is to boost the systems default
socket memory limits during a testrun.  This behaviour does not
always result in improved performance, because this will stress
the kernels SLAB allocators unnecessary.

Introducing an option "--no-sock-mem" that instruct trafgen
to not perform any socket memory adjustments.

Fixes: #130
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>