cleartext indexing version 5

[Daniel Kahn Gillmor]

Thanks to everyone for their reviews of this series that provides
indexing of the cleartext of encrypted messages (and thanks to bremner
for already pushing some of the less controversial patches of the
previous revisions of this series).  I hope you'll agree that your
suggestions have improved the propsoal :)

In particular, the following significant changes have been made since
the previous revision:

 * all properties whose key is prefixed with "index." are cleared (and
   possibly re-set) upon reindexing, not just a registered list of
   "autoproperties".

 * handling of command-line options for indexing (i.e. for the "new",
   "reindex", and "insert" subcommands) has been consolidated into a
   centralized location, which not only reduces total code footprint
   but will make it easier to add shared indexing options in the
   future.

I welcome review and feedback.

      --dkg

Series History
--------------

the first version of this series was sent starting at:

id:20170912230153.4175-1-dkg at fifthhorseman.net

version 2 of this series was sent as:

id:20170915055359.24123-1-dkg at fifthhorseman.net

version 3 of this series was sent as:

id:20171010054916.23925-1-dkg at fifthhorseman.net

and version 4 was only partially sent to the list.