add more golangci-lint checks

.golangci.yml

@@ -1,6 +1,24 @@
 linters:
   enable:
+    - dogsled
+    - dupl
     - goimports
-    - stylecheck
+    - gosec
     - misspell
+    - nakedret
+    - stylecheck
+    - unconvert
+    - unparam
     - whitespace
+
+issues:
+  exclude-rules:
+    # Some cache implementations use md5 hashes for cached filenames.  There is
+    # a slight risk of cache poisoning if an attacker could construct a URL
+    # with the same hash, but it would also need to be allowed by the proxies
+    # security settings.  Changing these to a more secure hash algorithm would
+    # result in 100% cache misses when users upgrade. For now, just leave these
+    # alone.
+    - path: internal/.*cache
+      linters: gosec
+      text: G(401|501)