golangci-lint: Upgrade, fix issues (#397)

The golangci-lint being used was quite dated.
This change upgrades to the latest version.
Adds and updates exclusions based on new failures.

Note on revive:
I've included an opt-out for unused parameters for revive
because turning `newThing(required bool)` to `newThing(_ bool)`
is a loss of useful information.

The changes to the Go files are to fix the following issues:

```
camelcase.go:16: File is not `gofmt`-ed with `-s` (gofmt)
config_test.go:50:18: directive `//nolint: gosec` is unused for linter "gosec" (nolintlint)
defaults_test.go:28:25: G601: Implicit memory aliasing in for loop. (gosec)
doc.go:5: File is not `gofmt`-ed with `-s` (gofmt)
kong.go:446:18: directive `//nolint: gosec` is unused for linter "gosec" (nolintlint)
kong_test.go:503:20: G601: Implicit memory aliasing in for loop. (gosec)
model.go:493:10: composites: reflect.ValueError struct literal uses unkeyed fields (govet)
scanner.go:112: File is not `gofmt`-ed with `-s` (gofmt)
```

And to address broken nolint directives reported as follows by
golangci-lint.

```
[.. skipped .. ]
tag.go:65:1: directive `// nolint:gocyclo` should be written without leading space as `//nolint:gocyclo` (nolintlint)
tag.go:206:51: directive `// nolint: gocyclo` should be written without leading space as `//nolint: gocyclo` (nolintlint)
util_test.go:23:43: directive `// nolint: errcheck` should be written without leading space as `//nolint: errcheck` (nolintlint)
util_test.go:51:22: directive `// nolint: errcheck` should be written without leading space as `//nolint: errcheck` (nolintlint)
```

options_test.go

@@ -30,7 +30,7 @@ func TestBindTo(t *testing.T) {
 
 	saw := ""
 	method := func(i iface) error {
-		saw = string(i.(impl)) // nolint
+		saw = string(i.(impl)) //nolint
 		return nil
 	}
 
@@ -50,7 +50,7 @@ func TestInvalidCallback(t *testing.T) {
 
 	saw := ""
 	method := func(i iface) string {
-		saw = string(i.(impl)) // nolint
+		saw = string(i.(impl)) //nolint
 		return saw
 	}
 
@@ -75,7 +75,7 @@ func TestCallbackCustomError(t *testing.T) {
 
 	saw := ""
 	method := func(i iface) *zrror {
-		saw = string(i.(impl)) // nolint
+		saw = string(i.(impl)) //nolint
 		return nil
 	}