WIP: support gzipped message files

[David Bremner]

It turned out neither of the approaches I suggested really supported a
clean API. On the other hand, by adding some new functions to
util/gmime-extra.c, I've managed to avoid too much code duplication.
This version seems to be working, but it needs a few more tests and
the usual polishing. Feedback on the g_mime_stream_gzfile_* internal
api is particularly welcome. Looking at mime_node_open, there might be
a slight performance improvement to use open instead of fopen (because
gmime is going to buffer anyway). So it might make more sense to have
g_mime_stream_gzfile_new take a file descriptor.