ns_query refactoring for hook-based recursion

ns_query refactoring for hook-based recursion

several small changes to query processing to make it easier to
use hook-based recursion (and other asynchronous functionlity)
later.

- recursion quota check is now a separate function,
  check_recursionquota(), which is called by ns_query_recurse().
- pass isc_result to query_nxdomain() instead of bool.
  the value of 'empty_wild' will be determined in the function
  based on the passed result.  this is similar to query_nodata(),
  and makes the signatures of the two functions more consistent.
- pass the current 'result' value into plugin hooks.