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>

integration/main_test.go

@@ -509,14 +509,14 @@ func PidEnvs(pid int) (map[string]string, error) {
 
 	res := make(map[string]string)
 	for _, value := range values {
-		value := strings.TrimSpace(string(value))
+		value = bytes.TrimSpace(value)
 		if len(value) == 0 {
 			continue
 		}
 
-		parts := strings.SplitN(value, "=", 2)
-		if len(parts) == 2 {
-			res[parts[0]] = parts[1]
+		k, v, ok := strings.Cut(string(value), "=")
+		if ok {
+			res[k] = v
 		}
 	}
 	return res, nil