Support truncation

[Bouke]

UDP DNS messages are limited to 512 bytes. If messages are over 512 bytes, the message is cut-off at 512 bytes and the truncation bit is set. Currently the bit is parsed, but the message isn't handled differently, resulting in a parsing failure. The correct way here would be parsing only a partial message without throwing.

The other way around should also be supported, cutting off messages when truncation is supported.

Also, I need to read up on the spec to see how truncation actually works.