cmd/derper: add --stun-port flag

And flesh out docs on the --http-port flag.

Change-Id: If9d42665f67409082081cb9a25ad74e98869337b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f