v3 of legacy-display cleanup

[Daniel Kahn Gillmor]

This is the third revision of the series that cleans up legacy-display
protected headers parts so that notmuch users only have to look at one
subject line.

version 2 can be found at id:20190531075907.17035-1-dkg at fifthhorseman.net
version 1 can be found at id:20190531042825.27774-1-dkg at fifthhorseman.net

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Now that notmuch can handle and interpret protected subject lines, it
should also avoid forcing the user to look at "legacy display" parts
that some MUAs (notably enigmail) copies of the protected headers that
are intended to be rendered only by legacy clients -- clients capable
of decryption but which don't understand how to handle protected
headers.
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The main differences from version 2 are:

 * i've directly included the two-patch "repair" series (originally
   found at id:20190531074027.16337-1-dkg at fifthhorseman.net), and

 * i've rebased against the uncrustified master, as well as applying
   the uncrustify rules to my changes here.

If we can get this merged, i'll send a subsequent revision of the
series that repairs "mixed-up MIME" mangled messages.

I would appreciate any feedback!

  --dkg