make: Fix erl -> beam rule after renaming hrl file
Add dnsutils module, with helper functions

Currently only a function for converting a "on the wire" domain
to a human readable string representation (e.g. "ethup.se.").
dnspkt.hrl: add default rdatalen value of -1

This should be read as "calculate it yourself when needed". This
is to make it easier creating RRs (at least initially during
prototyping). I haven't gotten around actually using this, but
hopefully this approach is reasonable.