FUSE: Consider using "auto_unmount" in fuse::mount

[nivekuil]

Hey, thanks for writing up these tutorials. I found that using the auto_unmount option with FUSE was a real timesaver in development, since it lets you ^c the program and then re-run it without running a separate fusermount -u <path> in between. It doesn't seem to be documented very well and this is probably the best introduction to the crate, so it might be nice to demonstrate its use in this book.

fuse::mount(JsonFilesystem, &mountpoint, &["-o,auto_unmount".as_ref()]);