TODO items.
o use real entropy to make random (ID, port) numbers more random.
o in production mode, do not free memory on exit. In debug mode, test leaks.
o profile memory allocation, and if performance issues, use special memory
  allocator. For example, with caches per thread.
o do not malloc query_info qname of query for answers from cache. Use ptr
  to incoming commpt buffer.
o sendmsg, writev.