gmime 3.0 related preliminaries

[David Bremner]

I've been working away on porting to gmime 3.0 [1]; currently more
than 50 tests fail, at least some of which due to [2].

I've also found a few bugs in notmuch:

We currently use GmimeFileStream to output to stdout, but this assumes
the underlying file descriptor is seekable; the seek errors are masked
in gmime 2.6 but not in gmime 3.0

[PATCH 1/5] util: convenience function to create gmime stream for
[PATCH 2/5] cli/reply: direct all output for text format to gmime
[PATCH 3/5] cli/show: use single stream for printf / gmime object

There is also memory leak in notmuch reply; because notmuch-reply
doesn't run for a long time, the practical impact is low. The first
patch is just so we have some way of verifying the fix. It seems a bit
heavy to start using valgrind in the main test suite.


[1]: http://pivot.cs.unb.ca/git?p=notmuch.git;a=shortlog;h=refs/heads/gmime-3.0
[2]: https://bugzilla.gnome.org/show_bug.cgi?id=782915