Do not allow out-of-zone data for DDNS.

- RFC 2136 states that the server should return the 'NOTZONE' error
  code, which it is correcly doing now, so there's no need to change it.
- Minor change only, since the actual check is somewhere before this code
  executes, but I've left the new_node functions as it was, since we
  may wish to allow some malformations later.

Refs #102