Bug #14448: delimiter in 'prompt'

"I want to have the current delimiter in the prompt so that I can know at 
a glance which is set."

Add a 'l' format specifier that represents the current statement delimiter.

client/mysql.cc

@@ -3724,6 +3724,9 @@ static const char* construct_prompt()
       case 't':
 	processed_prompt.append('\t');
 	break;
+      case 'l':
+	processed_prompt.append(delimiter_str);
+	break;
       default:
 	processed_prompt.append(c);
       }