[Github Issue] Resolver hotplug hook
#### **DO NOT EDIT HERE!** Copy from https://github.com/CZ-NIC/turris-os-packages/issues/13
When an upstream interface get up later than a resolver, user preferred upstream forwarding of DNS queries is not followed. This PR fixes it both for
knot-resolver and for
unbound (where it used to work before migrating to
resolver-config unified configuration).
Original bug report: https://forum.turris.cz/t/dns-forwarding-stops-working/1817
Not thoughtfully tested as my WAN starts early. But it should work.