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)
```

.golangci.yml

@@ -41,10 +41,21 @@ linters:
     - exhaustruct
     - nonamedreturns
     - nilnil
+    - nosnakecase # deprecated since v1.48.1
+    - structcheck # deprecated since v1.49.0
+    - deadcode    # deprecated since v1.49.0
+    - varcheck    # deprecated since v1.49.0
+    - depguard    # nothing to guard against yet
+    - tagalign    # hurts readability of kong tags
 
 linters-settings:
   govet:
     check-shadowing: true
+    # These govet checks are disabled by default, but they're useful.
+    enable:
+      - niliness
+      - sortslice
+      - unusedwrite
   dupl:
     threshold: 100
   gocyclo:
@@ -66,3 +77,14 @@ issues:
     - 'bad syntax for struct tag pair'
     - 'result .* \(error\) is always nil'
     - 'package io/ioutil is deprecated'
+
+  exclude-rules:
+    # Don't warn on unused parameters.
+    # Parameter names are useful for documentation.
+    # Replacing them with '_' hides useful information.
+    - linters: [revive]
+      text: 'unused-parameter: parameter \S+ seems to be unused, consider removing or renaming it as _'
+
+    # Duplicate words are okay in tests.
+    - linters: [dupword]
+      path: _test\.go