4501: FUSE exposes by_id subdirectory when run with --by-id.

4501: FUSE exposes by_id subdirectory when run with --by-id.

This makes it possible for people to develop code against their own
normal Keep mount, and then have it work as expected in Crunch (which
uses --by-id for backward compatibility).

I considered adding a second inode for by_id that pointed to the same
underlying MagicDirectory instance.  Unfortunately, the current
implementation would make it difficult to avoid exposing infinitely
recursing by_id subdirectories, and that seemed like a bigger problem
than the relatively small overhead of having two MagicDirectory
instances.  (The Keep client will use comparatively more RAM, and it
will use the same block cache for both directories, which should keep
it relatively under control.)