Add constant for NXDOMAIN

[nhnt11]

Trivially OR'ing 0x03 to the flags works to set NXDOMAIN in the response header. Exposing a constant for this makes it accessible and obvious.

[silverwind]

I think instead of exporting this one rcode, we could instead export the toRcode function to make them all accessible. Maybe name it stringToRcode to also allow the reverse to be cleanly exported as rcodeToString.