Make PackRR enforce RR data length limit

• Make rawSetRdlength return false if length would overflow
• Make PackRR assume rawSetRdlength returning false indicates an overflow
Merge pull request #92 from andrewtj/atj-oversizerrdata

Make PackRR enforce RR data length limit
Add IsMsg()

A quick validating function that checks if a buffer is a valid
DNS message.
The code is *far* from complete.