all: simplify code by using modern Go constructs

Generated using modernize by running:

    go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

Change-Id: Ifc7d61cf6735cc53f2bdf890a338961f55075af5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/661975
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

unix/dirent_test.go

@@ -107,7 +107,7 @@ func TestDirentRepeat(t *testing.T) {
 	d := t.TempDir()
 
 	var files []string
-	for i := 0; i < N; i++ {
+	for i := range N {
 		files = append(files, fmt.Sprintf("file%d", i))
 	}
 	for _, file := range files {