Return Notsup for absolute symlinks instead of panicking

Generally when userspace tries to do something the environment can't do (even if it might be able to do it in the future), the usual response is the return a "Not Supported" error, rather than crashing the entire environment.

lib/wasix/src/fs/mod.rs

@@ -930,7 +930,8 @@ impl WasiFs {
                                 let (pre_open_dir_fd, relative_path) = if link_value.is_relative() {
                                     self.path_into_pre_open_and_relative_path(&file)?
                                 } else {
-                                    unimplemented!("Absolute symlinks are not yet supported");
+                                    tracing::error!("Absolute symlinks are not yet supported");
+                                    return Err(Errno::Notsup);
                                 };
                                 loop_for_symlink = true;
                                 symlink_count += 1;