-FIXME: A response type of "yn" should allow the user to specify
-"y" or "n" in either uppercase or lowercase. This is especially
-true since the message shown to the user will contain uppercase
-characters as valid choices.
-
-FIXME: If the response type is "free", the user cannot specify
-an empty string; showmessage will return "1" as the result.
-
-FIXME: A default response of "0" cannot be specified. This is
-wrong; the default response should be checked for undef, not
-for 0.
-
-FIXME: If $noclear is not specified or specified as undef, we
-just test it for a non-zero value without testing it for being
-undef first.
-