v4 of repairing Mixed-up mangled MIME messages

[Daniel Kahn Gillmor]

This is the fourth revision of the "Mixed up Mangling" series.
Previous versions:

 * v1 starts at id:20190528225452.17550-1-dkg at fifthhorseman.net
 * v2 starts at id:20190530172707.10378-1-dkg at fifthhorseman.net
 * v3 starts at id:20190531074842.16789-1-dkg at fifthhorseman.net

This is a simple rebase of v3 on top of the current notmuch master
branch.

Comments welcome, as always!

         --dkg