* Properly track isFile, isSymlink, isDirectory
These don't exhaust all the possibilities, so none of them should be
defined as "neither of the others".
* empty
There's a lot of variation in doc comments and internal code about
whether chmod/0o777-style permissions are called `mode` or `perm`. (For
example, mkdir and writeFile choose differently.)
Had proposed earlier to go consistently with `perm`, but on balance devs
prefer to go with `mode`.
