Add X-Dist response header with the distribution name

[rhoerr]

Description (*)

This adds a X-Dist header that outputs the distribution name (Mage-OS).

This is a reliable, non-invasive, and performant way of flagging installations for such purposes as https://github.com/HTTPArchive/wappalyzer.

I chose X-Dist to avoid conflicts with X-Powered-By and X-Built-With as used by various other systems.

Related Pull Requests

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Fixed Issues (if relevant)

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Manual testing scenarios (*)

1. Load homepage
2. Look at headers and see X-Distro: Mage-OS

Questions or comments

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Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)