v2 of Safe and useful handling of "Mixed Up" mangled messages

[Daniel Kahn Gillmor]

This is the second revision of the series initially posted at
id:20190528225452.17550-1-dkg at fifthhorseman.net.

The changes in this series from v1 are in response to the helpful
review by jrollins.  In particular:

 * test to ensure that 'notmuch reply' is fixed as well
 
 * the repair functionality moves from util/crypto.[ch] into its own
   source file at util/repair.[ch]

I welcome further review, comments, or suggestions.

Regards,

        --dkg