Separate UDP GSO and GRO controls for Linux (--gso and --gro options) #2037

[roaoliver]

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• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  3.21
• Issues fixed (if any):
  Separate UDP GSO and GRO controls for Linux (--gso and --gro options) #2037
• Brief description of code changes (suitable for use as a commit message):
  Separate UDP GSO and GRO controls for Linux (--gso and --gro options)

[davidBar-On]

Suggest to set only gso, so --gso --gro will be equivalent to --gsro.

[davidBar-On]

Suggest to set only gro, so --gso --gro will be equivalent to --gsro.

[davidBar-On]

To reduce number of iperf3 options, instead of adding --gso and --gro it may be better to add a flag to --gsro. E.g --gsro/s will set only gso and --gsro/r will set only gro.

[roaoliver]

Suggestion implemented in the code.