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 ([1], [2], [3]).
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[46].dns-priority" setting.
