smtpsrv: Quote unknown commands for debugging

When we receive unknown commands, we use the first 6 bytes for
troubleshooting (e.g. put them in traces and exported metrics).

While this is safe, since the different places know how to quote them
properly, it makes things more difficult to analyse, since it's not
uncommon to see be binary blobs.

This patch makes us use the ascii-quoted version instead, to make things
easier to analyze.

internal/smtpsrv/conn.go

@@ -278,7 +278,7 @@ loop:
 		default:
 			// Sanitize it a bit to avoid filling the logs and events with
 			// noisy data. Keep the first 6 bytes for debugging.
-			cmd = fmt.Sprintf("unknown<%.6s>", cmd)
+			cmd = fmt.Sprintf("unknown<%.6q>", cmd)
 			code = 500
 			msg = "5.5.1 Unknown command"
 		}