Windows / macOS: simplify code

1. simplify resource management by using C++ RAII like GCC attribute extension `cleanup`
2. simplify HEKY reading

Actually `__attribute__((__cleanup__(cleanupFn)))` is supported on all major C compiler ( gcc, clang and icc ) except MSVC I only used it on Windows and macOS for now. Let's see if the project author like it.

src/util/FFstrbuf.h

@@ -301,4 +301,9 @@ static inline FF_C_NODISCARD bool ffStrbufEndsWithIgnCase(const FFstrbuf* strbuf
 {
     return ffStrbufEndsWithIgnCaseNS(strbuf, end->length, end->chars);
 }
+
+#if defined(_WIN32) || defined(__APPLE__)
+    #define FF_STRBUF_AUTO_DESTROY FFstrbuf __attribute__((__cleanup__(ffStrbufDestroy)))
+#endif
+
 #endif