replace strings.Split(N) for strings.Cut() or alternatives

Go 1.18 and up now provides a strings.Cut() which is better suited for
splitting key/value pairs (and similar constructs), and performs better:

```go
func BenchmarkSplit(b *testing.B) {
        b.ReportAllocs()
        data := []string{"12hello=world", "12hello=", "12=hello", "12hello"}
        for i := 0; i < b.N; i++ {
                for _, s := range data {
                        _ = strings.SplitN(s, "=", 2)[0]
                }
        }
}

func BenchmarkCut(b *testing.B) {
        b.ReportAllocs()
        data := []string{"12hello=world", "12hello=", "12=hello", "12hello"}
        for i := 0; i < b.N; i++ {
                for _, s := range data {
                        _, _, _ = strings.Cut(s, "=")
                }
        }
}
```

    BenchmarkSplit
    BenchmarkSplit-10            8244206               128.0 ns/op           128 B/op          4 allocs/op
    BenchmarkCut
    BenchmarkCut-10             54411998                21.80 ns/op            0 B/op          0 allocs/op

While looking at occurrences of `strings.Split()`, I also updated some for alternatives,
or added some constraints; for cases where an specific number of items is expected, I used `strings.SplitN()`
with a suitable limit. This prevents (theoretical) unlimited splits.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cmd/ctr/commands/commands.go

@@ -230,13 +230,10 @@ func ObjectWithLabelArgs(clicontext *cli.Context) (string, map[string]string) {
 func LabelArgs(labelStrings []string) map[string]string {
 	labels := make(map[string]string, len(labelStrings))
 	for _, label := range labelStrings {
-		parts := strings.SplitN(label, "=", 2)
-		key := parts[0]
-		value := "true"
-		if len(parts) > 1 {
-			value = parts[1]
+		key, value, ok := strings.Cut(label, "=")
+		if !ok {
+			value = "true"
 		}
-
 		labels[key] = value
 	}
 
@@ -247,11 +244,11 @@ func LabelArgs(labelStrings []string) map[string]string {
 func AnnotationArgs(annoStrings []string) (map[string]string, error) {
 	annotations := make(map[string]string, len(annoStrings))
 	for _, anno := range annoStrings {
-		parts := strings.SplitN(anno, "=", 2)
-		if len(parts) != 2 {
+		key, value, ok := strings.Cut(anno, "=")
+		if !ok {
 			return nil, fmt.Errorf("invalid key=value format annotation: %v", anno)
 		}
-		annotations[parts[0]] = parts[1]
+		annotations[key] = value
 	}
 	return annotations, nil
 }