core/dns: emit CONFIG_CHANGED signal even if caching
With systemd-resolved, NetworkManager considers /etc/resolv.conf
unmanaged. This breaks hostname lookups in a subtle way: when a new
connection comes online, NM will initiate the hostname lookup before
propagating DNS updates to systemd-resolved, which of course will cause
the request to fail. And because NM doesn't update /etc/resolv.conf
,
it doesn't emit a CONFIG_CHANGED
signal which would've restarted the
lookup.
Fix this by emitting a signal also when using a caching DNS plugin.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1933863