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 #define BIT_... different on bigendian and smallendian systems so that
  the htons on flags is not needed to send a message from the cache.
o speed up pkt domain name decompression loop detection using counter perhaps.
o detect OS/400 pthreads implementation that allows upgrading to writelock
  on pthreads rwlocks and use it to examine-rd before storing-wr rrset cache.
o understand synthesized DNAMEs, so those TTL=0 packets are cached properly.
o understand NSEC/NSEC3, aggressive negative caching, so that updates to
  NSEC/NSEC3 will result in proper negative responses.
o fallback without EDNS if result is NOTIMPL, now only on FORMERR like in java.
o scrubber has slow pkt_subdomain and pkt_strict_subdomain functions.
o get serverselection algorithm out of local optimum.
  make subtargets to get rtt info for a couple of targets, like fetch-policy.
  or send out multiple queries to multiple servers.
o donotqueryaddresses allow specification of subnets, use trie to store.
  extend default list to contain illegal ip4 and ip6 addresses.
o memory profile; and if needed put serviced queries under qstate->region
  and special purpose region code, reuse blocks, shrink if too big.
o check query, option to enforce presence of qdsection in noerror answers.
o configuration option where port 53 is used for send and receive, no other
  ports are used.
o prime sequence in multiple queries, if glue missing from prime sequence.
o (option) to not send replies to clients after a timeout of (say 5 secs) has
  passed, but keep task active for later retries by client.
o private TTL feature
o pretend-dnssec-unaware, and pretend-edns-unaware modes for debug/workshops.
o delegpt use rbtree for ns-list, to avoid slowdown for very large NS sets.
o be able to have different listen and query-to addresses to bind to,
  so you can listen to localhost and query-to to the internet.