benchmarks/iperf: optionally use default port 5201, if available

[gpfountz]

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Describe the problem

iperf3 default port is 5201. But the plugin always uses a random port.

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Describe the proposed solution

Add an option: Use the standard iperf3 port 5201 when it is available; otherwise, use a random available port.

Might require an iPerf restart after applying this update.

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Related issue

None