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/dns/run.go

@@ -31,7 +31,7 @@ func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 		unboundCancel := func() { waitError <- nil }
 		closeStreams := func() {}
 
-		for l.GetSettings().Enabled {
+		for *l.GetSettings().DoT.Enabled {
 			var err error
 			unboundCancel, waitError, closeStreams, err = l.setupUnbound(ctx)
 			if err == nil {
@@ -54,7 +54,7 @@ func (l *Loop) Run(ctx context.Context, done chan<- struct{}) {
 			l.logAndWait(ctx, err)
 		}
 
-		if !l.GetSettings().Enabled {
+		if !*l.GetSettings().DoT.Enabled {
 			const fallback = false
 			l.useUnencryptedDNS(fallback)
 		}