Initial import of libdpf.

Co-authored-by: Cursor <cursoragent@cursor.com>

thirdparty/portable-snippets/once/README.md

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+# Once
+
+This is a simple abstraction for a once which will choose from a few
+different back-ends:
+
+ * If C11 threads are available, use `call_once()`.
+ * On Windows, use
+   [InitOnceExecuteOnce()](https://msdn.microsoft.com/en-us/library/windows/desktop/ms683493(v=vs.85).aspx)
+ * If `PTHREAD_ONCE_INIT` is defined (*i.e.*, if `<pthread.h>` has
+   been included prior to including `once.h`), use `pthread_once()`.
+ * Use `[atomic.h](../atomic)`.