cmd/tailscale: translate old serve commands to new ones

This PR fixes the isLegacyInvocation to better catch serve and
funnel legacy commands. In addition, it now also returns a string
that translates the old command into the new one so that users
can have an easier transition story.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>
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