v2 of skipping over legacy-display protected header parts

[Daniel Kahn Gillmor]

This is the second revision of the series that skips over
"legacy-display" protected header parts.

v1 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 difference from version 1 is that this series now depends on
the two-part series "Setup for message repair", found at
id:20190531074027.16337-1-dkg at fifthhorseman.net, and should be
somewhat easier to merge with v3 of the "mixed-up message mangling
repair" series.  (previous versions of the "legacy display" and "mixed
up" series could fail in subtle ways when merged in the wrong order)

Comments welcome!

         --dkg