[multicast_dns] Rewrite _readFQDN to avoid recursion

[andrewkolos]

Speculatively fixes flutter/flutter#155433. Read one of my later comments on that thread for my current understanding of this issue. In summary, this PR rewrites _readFQDN to avoid recursion.

For reviewers: For a simplified diff, see the first commit, change. This commit avoids formatting make sure the diff only highlights changes to logic rather than formatting.

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[bkonyi]

This change LGTM overall, but I'm wondering if it's just going to cause us to get into an infinite loop rather than a stack overflow, which is going to be much harder to identify.

Since we can't catch the stack overflow error, I wonder if it's worth adding a top-level debug property here that contains the packet that's currently being processed. Then, the tool can add use a timer to determine if we're spending too much time processing the MDNS response to dump a base64 encoded packet to the terminal along with a message asking users to file a bug and share the packet contents with us (assuming they're comfortable sharing it since it could possibly contain PII in the domain). WDYT?