[th/dns-resolved-fix-exclusive] dns: fix handling default routing domains with systemd-resolved
We used to set "~." domains for all devices that should be used for resolving unknown domains. Systemd-resolved also supports setting "SetLinkDefaultRoute()". We should only set the wildcard domain if we want that this interface is used exclusively. Otherwise, we should only set DefaultRoute. See (, , ). Otherwise the bad effect is if other components (wg-quick) want to set exclusive DNS lookups on their link. That is achieved by explicitly adding "~." and that is also what resolved's `/usr/sbin/resolvconf -x` does. If NetworkManager sets "~." for interfaces that are not important and should not be used exclusively, then this steals the DNS requests from those external components. In NetworkManager we know whether a link should get exclusive lookups based on the "ipv.dns-priority" setting.