Dropping GMime 2.6 compatibility layer

[Daniel Kahn Gillmor]

This series follows David Bremner's
id:20190501104643.5836-1-david at tethera.net removal of gmime 2.6.  It
adopts GMime 3.0's API and idiom throughout, removing any attempt to
paper over the differences between GMime 2.6 and 3.0.  All tests pass.

This is a nice simplification, dropping about 500 lines overall, and
reducing the amount of configuration and #ifdef'ery that we have to
reason about going forward.  I support this change, even though it
means i'll need to do a bit of rebasing work on my protected-headers
series to get that series to apply cleanly.  The simplification is
well worth it.

Summary:

 30 files changed, 70 insertions(+), 592 deletions(-)