test: l2fwd: add transmit completion support

Add new -C option for configuring packet transmit completion. Both poll
and event completions are supported at least partially: event completion
is currently supported only with scheduled queues.

Transmit completion can be configured to be attempted to be done for
every nth packet within limitations of the completion resources.

Signed-off-by: Tuomas Taipale <tuomas.taipale@nokia.com>
Reviewed-by: Matias Elo <matias.elo@nokia.com>