clean up dns_parse_callback

clean up dns_parse_callback

the only functional change here should be that MAXADDRS is only
checked for RRs that provide address results, so that a CNAME which
appears after an excessive number of address RRs does not get ignored.
I'm not aware of any servers that order the RRs this way, and it may
even be forbidden to do so, but I prefer having the callback logic not
be order dependent.

other than that, the motivation for this change is that the A and AAAA
cases were mostly duplicate code that could be combined as a single
code path.