FR: Silence setLinkDNS warnings when accept-dns=false is used

### What are you trying to do?

There are many scenarios in which host DNS should be left untouched.

From the docs it appears `tailscale up --accept-dns=false` should achieve it.

But it does not, the following entries continue to appear in the logs:

>
> health(warnable=dns-os): error: setLinkDNS: Permission denied
> health(warnable=dns): error: setLinkDNS: Permission denied

### How should we solve this?

`--accept-dns=false` should deactivate all aspects of setLinkDNS, including the healthchecks.

### What is the impact of not solving this?

_No response_

### Anything else?

_No response_

Start by tracing the setLinkDNS path and the handling of --accept-dns=false. Confirm where the dns-os and dns health warnings are emitted, then verify that using accept-dns=false produces no setLinkDNS warnings while DNS remains untouched.

Written by the indexing model from the issue text.