stub out set_oflags for unix i guess?

what do we do about unsupported stuff like that? silently return OK?

crates/wasi-c2/src/error.rs

@@ -34,6 +34,9 @@ pub enum Error {
     #[error("Unexpected IoError: {0}")]
     UnexpectedIo(#[source] std::io::Error),
 
+    #[error("Unsupported operation: {0}")]
+    Unsupported(String),
+
     // Below this, all variants are from the `$errno` type:
     /// Errno::TooBig: Argument list too long
     #[error("TooBig: Argument list too long")]