With systemd-resolved, NetworkManager considers
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
it doesn't emit a
CONFIG_CHANGED signal which would've restarted the
Fix this by emitting a signal also when using a caching DNS plugin.