dns: rework pending request-queue in NMDnsSystemdResolved
We had two separate queues, one for "SetLinkDNS" and one for "SetLinkDomains". Merge them into one, and track the operation as part of the new RequestItem structure. A visible change to before is that we now would make all requests per-interface first. Prevously, we would first make all SetLinkDNS requests (for all interfaces) and then all SetLinkDomains requests. It feels more correct to order the requests this way, not by type. The reason to merge is, that we will next get another operation and in the current scheme we would need 3 GQueue instances. While at it, refactor the code to use CList. We now anyway would need a new struct to track the operation, requiring to allocate and free it. Previously, we would only track the GVariant argument as data of the GQueue.
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