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)
```

context.go

@@ -161,7 +161,7 @@ func (c *Context) Empty() bool {
 }
 
 // Validate the current context.
-func (c *Context) Validate() error { // nolint: gocyclo
+func (c *Context) Validate() error { //nolint: gocyclo
 	err := Visit(c.Model, func(node Visitable, next Next) error {
 		switch node := node.(type) {
 		case *Value:
@@ -347,7 +347,7 @@ func (c *Context) endParsing() {
 	}
 }
 
-func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
+func (c *Context) trace(node *Node) (err error) { //nolint: gocyclo
 	positional := 0
 	node.Active = true
 
@@ -377,7 +377,7 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 				switch {
 				case v == "-":
 					fallthrough
-				default: // nolint
+				default: //nolint
 					c.scan.Pop()
 					c.scan.PushTyped(token.Value, PositionalArgumentToken)