Tidy up the WASI ErrorKind enum. (#5015)

* 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)`.

crates/wasi-common/src/snapshots/preview_1.rs

@@ -69,13 +69,9 @@ impl From<ErrorKind> for types::Errno {
     fn from(e: ErrorKind) -> types::Errno {
         use types::Errno;
         match e {
-            ErrorKind::WouldBlk => Errno::Again,
-            ErrorKind::Noent => Errno::Noent,
             ErrorKind::TooBig => Errno::TooBig,
             ErrorKind::Badf => Errno::Badf,
-            ErrorKind::Exist => Errno::Exist,
             ErrorKind::Ilseq => Errno::Ilseq,
-            ErrorKind::Inval => Errno::Inval,
             ErrorKind::Io => Errno::Io,
             ErrorKind::Nametoolong => Errno::Nametoolong,
             ErrorKind::Notdir => Errno::Notdir,
@@ -83,7 +79,6 @@ impl From<ErrorKind> for types::Errno {
             ErrorKind::Overflow => Errno::Overflow,
             ErrorKind::Range => Errno::Range,
             ErrorKind::Spipe => Errno::Spipe,
-            ErrorKind::NotCapable => Errno::Notcapable,
             ErrorKind::Perm => Errno::Perm,
         }
     }
@@ -261,7 +256,7 @@ impl TryFrom<std::io::Error> for types::Errno {
                 std::io::ErrorKind::NotFound => Ok(types::Errno::Noent),
                 std::io::ErrorKind::PermissionDenied => Ok(types::Errno::Perm),
                 std::io::ErrorKind::AlreadyExists => Ok(types::Errno::Exist),
-                std::io::ErrorKind::InvalidInput => Ok(types::Errno::Ilseq),
+                std::io::ErrorKind::InvalidInput => Ok(types::Errno::Inval),
                 _ => Err(anyhow::anyhow!(err).context(format!("Unknown OS error"))),
             },
         }