Increase niceness of CivetWeb threads

[DL6ER]

What does this implement/fix?

Increase niceness of all CivetWeb threads as DNS operation is more important than the API and dashboard. This is a follow-up on #2305 to ensure the webserver workers cannot take over the system on heavy load.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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