Misc improvements to `net/dns' and `net/private/ip'.

* Some racketisms.

* Use explicit `in-list' etc in for loops.

* Remove some redundant requires from `net/dns'.

* Move all tests to `tests/net', including a new `tests/net/ip'.  In the
  future there's a plan to have things like stripped zos etc for
  distribution, but we're not there yet, and the net collection is
  already organized nicely so this also makes it more uniform.

* Include the dns tests in the main test file.
Avoid output in ftp tests.

Also add a note explaining why the output is not tested.