Updates tailscale/tailscale#13326
This PR adds another subcommand to `tailscale dns`, to stream queries and answers returned by the DNS forwarder as they are handled.
Useful for debugging purposes, and is equivalent to setting the `TS_DEBUG_DNS_FORWARD_SEND` envknob and filtering the logs for relevant entries. This also adds a new envknob, `TS_DEBUG_DNS_INCLUDE_NAMES`, which includes the actual hostnames in the log lines (with a huge privacy warning!). This makes it easier to diagnose issues with DNS resolution.
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