MINOR: dns: Update analysis of TRUNCATED response for SRV records

MINOR: dns: Update analysis of TRUNCATED response for SRV records

First implementation of the DNS parser used to consider TRUNCATED
responses as errors and triggered a failover to an other query type
(usually A to AAAA or vice-versa).

When we query for SRV records, a TRUNCATED response still contains valid
records we can exploit, so we shouldn't trigger a failover in such case.

Note that we had to move the maching against the flag later in the
response parsing (actually, until we can read the query type....)