fixestailscale/tailscale#18436
Queries can still make their way to the forwarder when accept-dns is disabled.
Since we have not configured the forwarder if --accept-dns is false, this errors out
(correctly) but it also generates a persistent health warning. This forwards the
Pref setting all the way through the stack to the forwarder so that we can be more
judicious about when we decide that the forward path is unintentionally missing, vs
simply not configured.
Testing:
tailscale set --accept-dns=false. (or from the GUI)
dig @100.100.100.100 example.com
tailscale status
No dns related health warnings should be surfaced.
Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>