* Tidy up the WASI `ErrorKind` enum.
`ErrorKind` is an internal enum used in wasi-libc to represent WASI
errors that aren't precisely represened by `std::io::ErrorKind` errors.
Add a descriptive comment, and remove some codes that are no longer
needed:
- Remove `NotCapable`, which is no longer used.
- Remove `WouldBlk`, `Exist`, `Noent`, and `Inval`, which have
one-to-one correspondences with codes in `std::io::ErrorKind`.
This will simplify the error handling in #4947 and #4967, as it means
the code will no longer have to check for two different forms of these
errors.
* Map `std::io::ErrorKind::InvalidInput` to `Ok(types::Errno::Inval)`.
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