lib/message_file: open gzipped files

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.