[PATCH v2] test: l2fwd: add transmit completion support

[TuomasTaipale]

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.

v2:

• Rebased
• Matias' comments