mkdir-immutable probably shouldn't implicitly create (mutable) intermediate directories

[davidsarah]

From #903:

... I don't think that automatically creating the intermediate directories makes a lot of sense for the mkdir-immutable-with-path operations:

• if the intermediate directories are created as immutable, then they will only ever have one child, and that isn't very useful.

• if they are created as mutable, that's irregular and might not be what the user (or programmer, if they're not paying close attention to docs) expected.

So I think that a mkdir-immutable-with-path should fail if the intermediate directories don't already exist (in which case, they must obviously be mutable). Explicit Is Better Than Implicit.

The current code creates the intermediate directories as mutable.