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/portforward/fs.go

@@ -6,7 +6,7 @@ import (
 )
 
 func (l *Loop) removePortForwardedFile() {
-	filepath := l.state.GetSettings().Filepath
+	filepath := *l.state.GetSettings().Filepath
 	l.logger.Info("removing port file " + filepath)
 	if err := os.Remove(filepath); err != nil {
 		l.logger.Error(err.Error())
@@ -14,7 +14,7 @@ func (l *Loop) removePortForwardedFile() {
 }
 
 func (l *Loop) writePortForwardedFile(port uint16) {
-	filepath := l.state.GetSettings().Filepath
+	filepath := *l.state.GetSettings().Filepath
 	l.logger.Info("writing port file " + filepath)
 	if err := writePortForwardedToFile(filepath, port); err != nil {
 		l.logger.Error(err.Error())