core: rework handling of default-routes and drop NMDefaultRouteManager
Remove NMDefaultRouteManager. Instead, add the default-route to the NMIP4Config/NMIP6Config instance. This basically reverts commit e8824f6a. We added NMDefaultRouteManager because we used the corresponding to `ip route replace` when configuring routes. That would replace default-routes on other interfaces so we needed a central manager to coordinate routes. Now, we use the corresponding of `ip route append` to configure routes, and each interface can configure routes indepdentently. In NMDevice, when creating the default-route, ignore @auto_method for external devices. We shall not touch these devices. Especially the code in NMPolicy regarding selection of the best-device seems wrong. It probably needs further adjustments in the future. Especially get_best_ip_config() should be replaced, because this distinction VPN vs. devices seems wrong to me. Thereby, remove the @ignore_never_default argument. It was added by commit bb750260, I don't think it's needed anymore. This brings another change. Now that we track default-routes in NMIP4Config/NMIP6Config, they are also exposed on D-Bus like regular routes. I think that makes sense, but it is a change in behavior, as previously such routes were not exposed there.
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