syscall: add Open O_* flags for Windows FILE_FLAG_* flags

// This program demonstrates the current limitation on Windows: the
// syscall package does not expose O_OVERLAPPED (or any FILE_FLAG_*
// flags) and therefore os.Open cannot open a file for overlapped I/O.
// Running this on Windows will simply open the file in the default
// mode and print any error that occurs.  The change described in the
// issue would allow passing O_OVERLAPPED to syscall.Open, but that
// constant is not available in Go 1.27.1.

package main

import (
	"fmt"
	"syscall"
)

func main() {
	// Attempt to open a file with the default read-only flag.
	// On Windows this will not request overlapped I/O.
	fd, err := syscall.Open("example.txt", syscall.O_RDONLY, 0)
	if err != nil {
		fmt.Printf("open error: %v\n", err)
		return
	}
	defer syscall.Close(fd)
	fmt.Println("file opened successfully (no overlapped flag available)")
}