NCR: change _ttl to signed integer (rfc1035).

* Set _name and _answer to NULL after deleted.
NCR_List: do not delete _rec when destroyed, let NCR_Tree do it.
NCR_Tree: fix red-black tree node removal.

* prune: delete _rec and set _p_list to NULL.
* ~NCR_Tree: set _plist to NULL.
NameCache: change parameter.

* use signed integer on NCR _ttl.
Rescached: pass question address to process_client and queue_push.
main: set running to 0 when quit.