Netlink tweaks

[DL6ER]

What does this implement/fix?

Following an issue report on Discourse, mentioning that not all interfaces were shown, I dived deeply into both the Netlink documentation as well as the kernel source code realizing that they are not always congruent. I modified our Netlink code in a way that would be consistent with how the kernel does it but still work if the way the kernel does it would be modified to behave as documented (I know this sounds a bit weird). In addition, we now have a lot of additional code that handles warning the user in case anything unexpected (most worth noticing would be Netlink packet truncation).

This PR adds the new config option debug.netlink obviously defaulting to false.

Extensively tested on three x86_64, a Raspberry Pi 1 B+, 4 and a 5 running the aarch64 kernel. On the latter, the problem was originally reported and I was able to reproduce it on this hardware.

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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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5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
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[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/interfaces-tab-only-shows-eth0-and-lo-interfaces/75338/22