[notmuch] Simple improvements to saving attachments

[Keith Amidon]

I found there were some attachments I couldn't save using the existing
functionality.  Since I was already touching the code I implemented a
rough cut at the "save all attachments" function Carl requested as
well.

I don't think this is the best code in the world but it gets the job
done for now and I expect it will get replaced with something much
better when some more thought has been put into the split in
mime-handling functionality between the frontend and the backend.

       --- Keith