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.
- in some cases it we were using `strings.Split()`, but _actually_ were trying to match
  a prefix; for those I replaced the code to just match (and/or strip) the prefix.

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

configuration/parser.go

@@ -23,7 +23,7 @@ func MajorMinorVersion(major, minor uint) Version {
 }
 
 func (version Version) major() (uint, error) {
-	majorPart := strings.Split(string(version), ".")[0]
+	majorPart, _, _ := strings.Cut(string(version), ".")
 	major, err := strconv.ParseUint(majorPart, 10, 0)
 	return uint(major), err
 }
@@ -35,7 +35,7 @@ func (version Version) Major() uint {
 }
 
 func (version Version) minor() (uint, error) {
-	minorPart := strings.Split(string(version), ".")[1]
+	_, minorPart, _ := strings.Cut(string(version), ".")
 	minor, err := strconv.ParseUint(minorPart, 10, 0)
 	return uint(minor), err
 }
@@ -89,8 +89,8 @@ func NewParser(prefix string, parseInfos []VersionedParseInfo) *Parser {
 	}
 
 	for _, env := range os.Environ() {
-		envParts := strings.SplitN(env, "=", 2)
-		p.env = append(p.env, envVar{envParts[0], envParts[1]})
+		k, v, _ := strings.Cut(env, "=")
+		p.env = append(p.env, envVar{k, v})
 	}
 
 	// We must sort the environment variables lexically by name so that