The code was assuming that the record type was
MX even though this could be false. Also adjusted
some code to also make it easier to test.
Note: the behavior when MX records are missing
is probably still not right.
Closes PR 13654
* 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.
