This is a file system that will composite multiple FUSE
filesystems, so you can have for example

 /dir1   -> one simple FUSE filesystem
 /dir2   -> another simple FUSE filesystem
 /config -> a FS interface to daemon configuration

Each of these directories should be RawFileSystem instances.  This
class takes care of the mapping between global inode ids and inode ids
of each sub-FUSE filesystem.

No files or directories may be created in the toplevel dir. Instead,
the daemon should issue AddFileSystem() and RemoveFileSystem()
methods internally.  This could be done in response to writes in a
/config directory.