Revised dump-restore patches

[David Bremner]

I haven't written user docs yet. I'm considering whether I can improve
this small task by doing the larger task of splitting the man page and
having notmuch help call man.

The code is cleaned up quite a bit, the bug fix for 8bit characters is
rolled into the series, and following a suggestion of Jani, restore
now auto-detects what format the input is in.

I left the default dump format as "sup" (old style) in this round. I'm
leary of breaking people's scripts, so my current proposal is
deprecate the sup format (as default) for a release or two.