Allow "order none" in "rrset-order" rules

Allow "order none" in "rrset-order" rules

named-checkconf treats the following configuration as valid:

    options {
        rrset-order {
            order none;
        };
    };

Yet, the above configuration causes named to crash on startup with:

    order.c:74: REQUIRE(mode == 0x00000800 || mode == 0x00000400 || mode == 0x00800000) failed, back trace

Add DNS_RDATASETATTR_NONE to the list of RRset ordering modes accepted
by dns_order_add() to allow "order none" to be used in "rrset-order"
rules.  This both prevents the aforementioned crashes and addresses the
discrepancy between named-checkconf and named.