Configure golangci-lint and fix warnings

.golangci.yml

@@ -0,0 +1,39 @@
+---
+output:
+  sort-results: true
+
+linter-settings:
+  goimports:
+    local-prefixes: git.cacert.org/cacert-gosigner
+  misspell:
+    locale: US
+    ignore-words:
+      - CAcert
+
+linters:
+  disable-all: false
+  enable:
+    - errorlint
+    - gocognit
+    - goconst
+    - gocritic
+    - gofmt
+    - goheader
+    - goimports
+    - golint
+    - gomnd
+    - gosec
+    - interfacer
+    - lll
+    - makezero
+    - misspell
+    - nakedret
+    - nestif
+    - nlreturn
+    - nolintlint
+    - predeclared
+    - rowserrcheck
+    - scopelint
+    - sqlclosecheck
+    - wrapcheck
+    - wsl