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)

internal/vpn/tunnelup.go

@@ -26,7 +26,7 @@ func (l *Loop) onTunnelUp(ctx context.Context, data tunnelUpData) {
 		}
 	}
 
-	if l.dnsLooper.GetSettings().Enabled {
+	if *l.dnsLooper.GetSettings().DoT.Enabled {
 		_, _ = l.dnsLooper.ApplyStatus(ctx, constants.Running)
 	}