net/dns: add a local Windows hosts-update control

Allow TS_DEBUG_DISABLE_HOSTS_FILE_UPDATES to opt out of Windows hosts entries locally, using the existing tailscaled-env.txt configuration mechanism. Keep the control-plane node attribute authoritative and preserve upstream behavior when the local option is unset or false.

Cover defaults, local opt-out, nil control knobs, and policy precedence with Windows tests. This change is limited to DNS configuration; no route changes or upstream route-fix cherry-pick are included.

Updates tailscale/tailscale#14327

Signed-off-by: Caleb Crome <caleb.crome@flyzipline.com>
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