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. nodejs#3076 will remove the
wrong error code exports.ADNAME.

PR-URL: nodejs#3051
Fixes: nodejs#3050
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

lib/dns.js

@@ -331,6 +331,7 @@ exports.NOTIMP = 'ENOTIMP';
 exports.REFUSED = 'EREFUSED';
 exports.BADQUERY = 'EBADQUERY';
 exports.ADNAME = 'EADNAME';
+exports.BADNAME = 'EBADNAME';
 exports.BADFAMILY = 'EBADFAMILY';
 exports.BADRESP = 'EBADRESP';
 exports.CONNREFUSED = 'ECONNREFUSED';