From SUSv3:

Any [standard output] field need not be meaningful in all
    implementations. In such a case a hyphen ('-') should be
    output in place of the field value

So have the `-O label' option print out the string "  -" if the
process has no label.

Approved by:	Silence from rwatson and green (when asked in March...)

bin/ps/print.c

@@ -774,7 +774,7 @@ out:
 		(void)printf("%-*s", v->width, string);
 		free(string);
 	} else
-		(void)printf("%-*s", v->width, "");
+		(void)printf("%-*s", v->width, "  -");
 	return;
 }