dns: add missing exports.BADNAME

Adds the documented but missing DNS error exports.BADNAME. This export
has been there before but got lost in a 2012 commit that added more
error codes. https://github.com/nodejs/node/pull/3076 will remove the
wrong error code exports.ADNAME.

PR-URL: https://github.com/nodejs/node/pull/3051
Fixes: https://github.com/nodejs/node/issues/3050


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>