feat: tlsmiddlebox experiment (#817)

See https://github.com/ooni/probe/issues/2124

internal/engine/experiment/tlsmiddlebox/syscall_cgo.go

@@ -0,0 +1,29 @@
+//go:build cgo && windows
+
+package tlsmiddlebox
+
+//
+// CGO support for SO_ERROR
+//
+
+/*
+#cgo windows LDFLAGS: -lws2_32
+
+#ifdef _WIN32
+#include <winsock2.h>
+#endif
+*/
+import "C"
+
+import "unsafe"
+
+// getErrFromSockOpt returns the errno of the SO_ERROR
+//
+// This is the CGO_ENABLED=1 implementation of this function, which
+// returns the errno obtained from the getsockopt call
+func getErrFromSockOpt(fd uintptr) int {
+	var cErrno C.int
+	szInt := C.sizeof_int
+	C.getsockopt((C.SOCKET)(fd), (C.int)(C.SOL_SOCKET), (C.int)(C.SO_ERROR), (*C.char)(unsafe.Pointer(&cErrno)), (*C.int)(unsafe.Pointer(&szInt)))
+	return int(cErrno)
+}