Support parallel UDP streams under Windows and Cygwin

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  3.10

• Issues fixed (if any):
  iperf3 UDP test functionality depends on SO_REUSEADDR implementation #472 / iperf3 -c 127.0.0.1 -i1 -t 9999 -b50M -u -p5003 -d -P2 -V failed #1161

• Brief description of code changes (suitable for use as a commit message):

Suggested change to allow UDP parallel streams under Windows (and Cygwin). The change is based on the approach suggested by @rossro, as described in this #472 comment.

The change adds an option to the Windows Server's -p parameter that defines the number of ports available starting with the defined port :
-p <port>/[<num of ports]

E.g -p 5201/2 specifies that ports 5201 and 5202 can be used for the UDP streams.

UPDATE (16-Apr-2023):

Updated to be fully backward compatible, including between Windows and other OS (including rebase to 3.13). This is by adding a UDP_CONNECT_REPLY_NEXT_PORT from a Windows server, to indicate that a different port should be used for the next UDP stream. The only combination that does not work is a Windows Server with older client version when multi UDP streams are used. However, this did not work anyway (just the error condition is changed).

[BoySanic]

If the merge conflicts are resolved, is there something preventing this from merging otherwise?

I would love to use iperf to test bidirectional udp on windows and this would allow for that, in theory.

[davidBar-On]

If the merge conflicts are resolved, is there something preventing this from merging otherwise?

I would love to use iperf to test bidirectional udp on windows and this would allow for that, in theory.

As Windows is not officially supported by iperf3, I don't know if and when the iperf3 team will merge this PR. In any case, I have rebased the changes to the latest iperf3 master branch (3.19.1+), including conflicts resolution, so you may build from this branch.