[feladat @ 246]
Improvement of DNS validation code. See also [wiki:Documentation documentation on standards]. Closes ticket:6, trailing dot not allowed in content field (it is now stripped automagically as PowerDNS doesnt want it). Closes ticket:40, error shown if priority field was empty and record type was not MX (prio field is now ignored if record type is not MX).
CHANGELOG
Please see <https://code.krikkit.nl/trac/poweradmin/log/trunk>.