throughput: print usage if no argument is given

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
througput: add link-layer mode

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
throughput: print usage message

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>