os, syscall: implement Stat and Lstat

File.Stat is left as a stub for now.

Tests are a bit stubbed down because os.ReadDir, os.Symlink, and t.TempDir are not yet (fully) implemented.
TODO: reimport tests from upstream as those materialize.

src/os/file_windows.go

@@ -56,3 +56,21 @@ func tempDir() string {
 func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
 	return -1, ErrNotImplemented
 }
+
+// isWindowsNulName reports whether name is os.DevNull ('NUL') on Windows.
+// True is returned if name is 'NUL' whatever the case.
+func isWindowsNulName(name string) bool {
+	if len(name) != 3 {
+		return false
+	}
+	if name[0] != 'n' && name[0] != 'N' {
+		return false
+	}
+	if name[1] != 'u' && name[1] != 'U' {
+		return false
+	}
+	if name[2] != 'l' && name[2] != 'L' {
+		return false
+	}
+	return true
+}