>     I think we should generalize EBADF to attempts to do
>     something fd-ish on a port without a fd.  So we raise on EBADF, but
>     otherwise return #t or #f depending on the result of isatty().

> ENOTSUP makes more sense.

My intuition says we shouldn't fabricate errno values for errors that
don't have them. But I don't have anything concrete to back it up.