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.