Added getSingleIndexedParamValue() that retrieves an index into a list of
values from the named query parameter, and then indexes into the supplied list
of values to return the corresponding value from the list. Used to convert
numeric index query parameters passed by POST to the GET redirect target, so
that the GET code can display messages, etc. (without passing the message
itself as a query argument and having to escape it or have silly people pass
in their own...).