Datatypes: literal should have value attribute

The literal element should have a value attribute that allows the user to
specify an integer used to represent the literal when using a datatype
sensitive parser.  Multiple literal elements in an enumeration should be
able to have the same integer value.

<datatype name="multilingualBoolean">
     <enumeration>
	<literal value="0">false</literal>
	<literal value="0">non</literal>
	<literal value="0">nien</literal>
	<literal value="1">true</literal>
	<literal value="1">oui</literal>
	<literal value="1">ya</literal>
     <enumeration>
</datatype>