BUG/MAJOR: dns: don't treat Authority records as an error

BUG/MAJOR: dns: don't treat Authority records as an error

Support for DNS Service Discovery by means of SRV records was enhanced with
commit 13a9232eb ("MEDIUM: dns: use Additional records from SRV responses")
to use the content of the answers Additional records when present.

If there are Authority records before the Additional records we mistakenly
treat that as an invalid response. To fix this, just ignore the Authority
section if it exist and skip to the Additional records.

As 13a9232eb was introduced during 2.2-dev, it must be backported to 2.2.
This is a fix for issue #778