[client] Always register NetBird with plain Linux DNS and use original servers as upstream

[lixmal]

Describe your changes

• Removes the restriction that only all-domains resolvers can be registered on Linux with resolvconf and file managers
• Registers NetBird as the sole nameserver
• Registers original nameservers as upstream with low priority: NetBird-assigned "all domains" resolvers can take precedence
• Replace DNS-related server IP strings with netip.Addr
• Use resolvconf as a fallback instead of openresolv
• Return RcodeRefused if no handler was found in the chain

Issue ticket number and link

Stack

Checklist

• [x] Is it a bug fix
• [ ] Is a typo/documentation fix
• [x] Is a feature enhancement
• [x] It is a refactor
• [ ] Created tests that fail without the change (if possible)
• [ ] Extended the README / documentation, if necessary

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[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
1 Accepted issue

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[buzzzo]

Please modity this by permit sysadmin to override it. It should cause issues on some scenarios.