HOST ToDo -- Things that could be fixed/improved/added.

Patches to close any of these items will be gratefully accepted!  Other
suggestions and comments are also encouraged.

- implement host_h_strerror() to return descriptions for extended error
  codes from get_info() -- or maybe don't over-load h_errno in
  get_hinfo() but instead also implement global host_errno?

- util.c:canonical() should not use geth_byname() but should only use
  the results from the get_info(T_A) so as to avoid accidentally seeing
  YP or /etc/hosts entries that would not actually be valid.  Note that
  fixing this involves makeing sure callers and users of the return
  status that get_info() stores in h_errno don't accidentally assume
  it's a valid input to h_strerror().  This will fix the problem with
  /etc/hosts entries causing fake "not canonical" errors when the wrong
  hostname appears in h_hname.

- do not use gethostbyname()/gethostbyaddr() or their newer replacements
  to do basic A and PTR lookups and checks [i.e. addr.c:check_addr() and
  addr.c:check_name()], but instead use the low-level interfaces.
  Provide separate options for using those functions just so that the
  native implementations can be tested.

- check that the names in the answer records match the names we asked
  for (taking CNAMEs into account where they're allowed).

- allow for setting the (stream) read timeout (conn_timeout) at runtime.

- use setservbyname() to get the local NAMESERVER_PORT value.

- allow runtime specification of destination port in server with ':PORT'

- allow runtime specification of source port with '-O'

- clean up the host_res_*() mess.

- don't strip the trailing dot from PTR targets that have only one label.

- maybe '-P' should be the default for '-C', but then we'd need
  something like a '--not-parent' or '--listed-ns' variant to explicitly
  check the nameservers listed inside the zone and it all gets a whole
  lot messier....

- check for both A and AAAA records.  The check for canonical hostnames
  in NS and MX records should be extended to include also AAAA records.
  Originally asked by Matti Aarnio <matti.aarnio@sonera.fi>

- try to build a regression test suite (but how do we feed it known
  bogus junk to test error handling?  nc?).

- think about using autobuild.
