Return `ENOTDIR` instead of `ENOTCAPABLE` when `*at` functions are given a non-directory as their base. This is in accordance with POSIX: https://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html Currently wasi-libc contains some code to detect such cases and rewrite `ENOTCAPABLE` to `ENOTDIR`, however it's better for WASI implementations to just do the right thing in the first place.