pg_get_functiondef forgets about most GUC_LIST_INPUT GUCs

Hi all,

While reviewing another patch related to the use of pg_strcasecmp in the
backend, I have noticed this bit in ruleutils.c:

    /*
     * Some GUC variable names are 'LIST' type and hence must not
     * be quoted.
     */
    if (pg_strcasecmp(configitem, "DateStyle") == 0
        || pg_strcasecmp(configitem, "search_path") == 0)
        appendStringInfoString(&buf, pos);
    else
        simple_quote_literal(&buf, pos);

However this fails to consider all GUCs marked as GUC_LIST_INPUT, like
the recent wal_consistency_checking. guc.c already holds a find_option()
which can be used to retrieve the flags of a parameter. What about using
that and filtering by GUC_LIST_INPUT? This requires exposing the
function, and I am not sure what people think about that.

Thoughts?
--
Michael