Checkpoint commit. - Add the majority of the list behavioral knobs. - Add some quick helpers that can make list changes safely (write, unlink, touch) - Fix tests.

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−return-path
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−return-receipt-to
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−content-length
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−precedence
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−x-confirm-reading-to
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−x-pmrqc
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−list-subscribe
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−list-unsubscribe
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−list-help
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−list-post
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−sender