ipn/policy: treat DNS service as interesting so it's not filtered out

The control plane is currently still eating it.

Updates #1713

Change-Id: I66a0698599d6794ab1302f9585bf29e38553c884
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f