ipn/ipnlocal: add new DNS and subnet router policies

In addition to the new policy keys for the new options, some
already-in-use but missing policy keys are also being added to
util/syspolicy.

Updates ENG-2133

Change-Id: Iad08ca47f839ea6a65f81b76b4f9ef21183ebdc6
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f