v3 of repairing Mixed-up mangled MIME messages

[Daniel Kahn Gillmor]

This is the third revision of the "Mixed up Mangling" series.  Version
1 was at id:20190528225452.17550-1-dkg at fifthhorseman.net.  Version 2
can be found at id:20190530172707.10378-1-dkg at fifthhorseman.net.

The main difference here 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 v2 of the "skipping legacy display"
series.  (previous versions of the "legacy display" and "mixed up"
series could fail in subtle ways when merged in the wrong order)

Comments welcome, as always!

         --dkg