Encode messages using IBDNS.DNS.Opcode conversion functions

Suppose the server needs to generate a DNS message with an opcode different from 0 (QUERY). There are atoms, such as :iquery, that stand for particular numeric values, much like what is done with resource record types. But the message-encoding codes did not call out to IBDNS.DNS.Opcode.to_int!() as it should, and therefore failed to encode dummy packets with atom-valued opcodes.

This fix does not address any user-visible bugs, but will be necessary later on for further developments.