dns/ddclient: Preserve the other address family (A/AAAA) on deSEC update (#5232)

[max-foss]

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Related issue
#5232

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Describe the problem
Using desec-v4 and desec-v6 alongside each other (dual-stack) causes either the A- or the AAAA-record to be deleted again.

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Describe the proposed solution
Here we add preserve for the other address family in line with the official documentation.

The behaviour for hostnames having either only an AAAA- or an A-record (single-stack) is unchanged.

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[AdSchellevis]

it's better to add a separate implementation when specific parameters are needed which are not part of the standard dyndns implementation.

[max-foss]

For a separate implementation to be worthwhile, we would need to touch the UI: Adding a checkbox to toggle the preserve behaviour (for the tiny minority who might want it), removing the unused username field, and replacing the password field with token in line with the deSEC naming scheme.

Otherwise we'd just be duplicating ~100 lines of code needlessly.

What do you think?

[max-foss]

Hi all,

I just pushed a proper full rework of this PR, I am already rocking it locally.

Review notes:

• The migration for existing entries keeps the pruning of the other address family enabled by default (as-is), while new entries disable it by default.
• Rollback to an earlier version is fully supported after updating, but not if the entry was created after the update to this version.
• The terminology is now fully aligned with the deSEC documentation; neither a username nor a password (in the literal sense of an account password) was ever supported, so this is not removing any actual functionality.

For the rest, please see my comments. I took great care writing them.

Cheers

Max