add support for PXO's NAT hole punch

[notimaginative]

Pull Request Type

• GitHub Workflow changes
• Documentation or Wiki changes
• Build and Dependency changes
• Runtime changes
  • Render changes
  • Audio changes
  • Input changes
  • Network changes
  • Other changes

Description

Adds support for PXO's NAT hole punch feature allowing game servers to be used on PXO without having to set up port mapping. Also works with some types of CG-NAT. The PXO tracker does the vast majority of the work needed for this feature, so client changes are minimal.

Related Issues

Screenshots (if applicable)

Checklist

• I have tested my changes locally and verified that they work as intended.
• I have documented any new or modified functionality.
• I have reviewed the changes to ensure they do not introduce any unnecessary complexity or duplicate code.
• I understand that by submitting this pull request, I am agreeing to license my contributions under the project's license.

Additional Comments

This feature has full IPv6 support, however since Descent3 does not yet support IPv6 those bits of the code have either been removed of commented out with an appropriate note. Adding this ability back once IPv6 is supported should be trivial.

A brief description of how this feature works was prepared for those who do FreeSpace multiplayer support so that they could better help anyone experiencing trouble:
PXO Nat Hole Punch.pdf

[Lgt2x]

Nice, I had no idea this hole punch technique was a thing :) Thanks for pointing the doc. I also recommend reading the Wikipedia page about it for anyone interested, especially the flow chart!