ipn/ipnlocal: replace magicDNSAddrs bool params with a flags type

Two adjacent bool arguments at call sites are easy to transpose and
hard to read. Use an unexported bitmask type instead, per review
feedback on PR #20414.

Updates #cleanup

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I0417270c9c3f4b2522911c39aad375cbaf025a82
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