chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

.golangci.yml

@@ -27,6 +27,14 @@ issues:
     - text: "mnd: Magic number: 0400*"
       linters:
         - gomnd
+    - text: "variable 'mssFix' is only used in the if-statement*"
+      path: "openvpnconf.go"
+      linters:
+        - ifshort
+    - text: "variable 'auth' is only used in the if-statement*"
+      path: "openvpnconf.go"
+      linters:
+        - ifshort
 linters:
   enable:
     # - cyclop