Fix bug in parsing of PTR, and add a test

Fixes PTR (#30) and AFSDB.

I've also changed the other calls to parse_name from parse_rdata to pass zero for "base", because passing anything other than zero is a bit misleading.

I still think that the "base" argument could probably be removed entirely to simplify the code, and parse_name could still return the length if there is some caller that uses it.