wasi-c2: implement a synchronous poll_oneoff in terms of yanix

doesnt work on unix yet! also breaks all the rules about using the
cap-std family instead of rawfds! but this is cool and im happy with it

crates/wasi-c2/src/table.rs

@@ -62,7 +62,7 @@ impl Table {
                 Err(Error::Exist) // Does exist, but borrowed
             }
         } else {
-            Err(Error::Exist) // Does not exist
+            Err(Error::Badf) // Does not exist
         }
     }