chore: import upstream snapshot with attribution

Sources/CShim/include/openat2.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright © 2026 Apple Inc. and the Containerization project authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef __OPENAT2_H
+#define __OPENAT2_H
+
+#include <sys/types.h>
+
+#ifndef RESOLVE_IN_ROOT
+#define RESOLVE_IN_ROOT 0x10
+#endif
+
+struct cz_open_how {
+  unsigned long long flags;
+  unsigned long long mode;
+  unsigned long long resolve;
+};
+
+/// openat2(2) wrapper. Musl does not provide openat2 so we invoke the syscall
+/// directly. Requires Linux 5.6+.
+int CZ_openat2(int dirfd, const char *pathname, struct cz_open_how *how,
+               size_t size);
+
+#endif