ipn/ipnlocal: restrict exit node DoH server based on ACL'ed packet filter

Don't be a DoH DNS server to peers unless the Tailnet admin has permitted
that peer autogroup:internet access.

Updates #1713

Change-Id: Iec69360d8e4d24d5187c26904b6a75c1dabc8979
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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