* Must always be non-null pointer;
* If *query_io is 0 to start with then any query may be returned;
* If *query_io is !0 adns_query then only that query may be returned.
+ * If the call is successful, *query_io, *answer_r, and *context_r
+ * will all be set.
* Errors:
* Return values are 0 or an errno value;
* Seriously fatal system errors (eg, failure to create sockets,