Miscellaneous small FUSE filesystems

* hello: Sample filesystem, from the original FUSE distribution
* hello_ll: Sample low-level filesystem, from FUSE 

* dup_ll: Mounts an exact copy of an existing directory
* many: FS with an enormous number of files
* big_ll: FS with a single huge multi-TB file

* tree_write: Outputs the structure of a dir. tree to a file
* tree_ll: Reads such a file, and mounts the directory

TODO
	- many should be low-level