This drops "file" from mime_node_context and just uses a local
variable. It also uses the new gzip aware utility routines recently
added to util/gmime-extra.c. The use of gzopen / gzfile in addition is
a bit icky, but the choice is between that, and providing yet another
readline implimentation that understands GMime streams.
Rather than storing the lower level stdio FILE object, we store a
GMime stream. This allows both transparent decompression, and passing
the stream into GMime for parsing. As a side effect, we can let GMime
close the underlying OS stream (indeed, that stream isn't visible here
anymore).
This change is enough to get notmuch-{new,search} working, but there is still
some work required for notmuch-show, to be done in a following commit.
