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/leases/leases.go

@@ -128,12 +128,8 @@ var createCommand = cli.Command{
 		if len(labelstr) > 0 {
 			labels := map[string]string{}
 			for _, lstr := range labelstr {
-				l := strings.SplitN(lstr, "=", 2)
-				if len(l) == 1 {
-					labels[l[0]] = ""
-				} else {
-					labels[l[0]] = l[1]
-				}
+				k, v, _ := strings.Cut(lstr, "=")
+				labels[k] = v
 			}
 			opts = append(opts, leases.WithLabels(labels))
 		}