system: "migrate" tunables config.xml defaults to newer automatic tunables framework

[fichtner]

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• I am convinced that my issue is new after having checked both open and closed issues at https://github.com/opnsense/core/issues?q=is%3Aissue

Is your feature request related to a problem? Please describe.

config.xml still has old entries with pseudo-default values which need to be recorded in system.inc anyway.

Describe the solution you like

Nowadays we have the required keyword which is the better approach to embedded config.xml defaults.

Describe alternatives you considered

N/A

Additional context

N/A

[fichtner]

Note to self: the new GUI has a weak spots:

• Seems to cache the tunables state even after apply (having changed them actually)
• The delete button shows for tunables that are not in the config