wasip3 filesystem unlink: Homogenize error returns

When attempting to unlink a directory, POSIX specifies the result should
be EPERM, but only MacOS implements that behavior.  Paper over the
differences.  Also turn EACCESS into EPERM, to paper over Windows
differences.

Related to WebAssembly/WASI#852 and
WebAssembly/wasi-testsuite#137.

POSIX 2008 reference: https://pubs.opengroup.org/onlinepubs/9699919799/functions/unlink.html

Author: wingo

crates/wasi/src/filesystem.rs

@@ -1132,8 +1132,24 @@ impl Dir {
             return Err(ErrorCode::NotPermitted);
         }
         self.run_blocking(move |d| d.remove_file_or_symlink(&path))
-            .await?;
-        Ok(())
+            .await
+            .map_err(|e| {
+                match ErrorCode::from(e) {
+                    // Linux returns EISDIR when attempting to unlink a
+                    // directory instead of the POSIX-specified EPERM.
+                    ErrorCode::IsDirectory => ErrorCode::NotPermitted,
+                    // Windows on the other hand returns EACCESS when
+                    // attempting to unlink a directory.  Unfortunately
+                    // filesystem ownership and permissions can also give
+                    // rise to EACCESS, on Windows and elsewhere.  However
+                    // give filestem permissions aren't part of WASI, we can
+                    // regain a single behavior if we reduce the precision
+                    // on these errors by mapping all EACCESS errors to
+                    // EPERM, on all platforms.
+                    ErrorCode::Access => ErrorCode::NotPermitted,
+                    e => e,
+                }
+            })
     }
 
     pub(crate) async fn metadata_hash_at(