Add new dump/restore format and batch tagging.

[david at tethera.net]

This is a patch series with some history, if you will forgive the
semi-inadvertant pun. I list that at that end, for the curious.

If this series goes in, in the future we might consider whether
restore --accumulate provides important functionality over batch
tagging; at the moment I suggest leaving it as the extra code to
support it is minimal, and it does support peoples existing scripts
using the old dump/restore format.

History
-------

About 1 year ago, Petter Reinholdtsen observed a problem with dumping
and restoring message-id's with spaces in them. 

    id:2flfwhht87d.fsf at diskless.uio.no

There followed a proposed fix

      id:1323808075-7417-1-git-send-email-david at tethera.net

Which Dmitry had a few helpful things to say about the hex encoding
libs.

Jani took that foundation and proposed two versions of the batch
tagging, most recently at

id:cover.1334404979.git.jani at nikula.org. 

There was some discussion with Jamie about the file format for batch
tagging in the thread

     id:cover.1333231401.git.jani at nikula.org

id:1323808075-7417-1-git-send-email-david at tethera.net
The first 6 of these patches obsolete the series

    id:1345382314-5330-1-git-send-email-david at tethera.net

which was revied by Tomi and Ethan. I think I implemented their
suggestions.

Although I didn't re-read that whole thread, I believe this version of
the patches address's Jamie's concerns by using exactly the same
format for restore and tag --batch (renamed from Jani's choice of
--stdin).