Ignore/discard DNS SRV response with truncated flag set

[riza]

The ​standard stated:

DNS resolvers and recursive servers MUST support UDP, and
SHOULD support TCP, for sending (non-zone-transfer) queries.
Specifically, a DNS resolver or server that is sending a
non-zone-transfer query MUST send a UDP query first.  If the
Answer section of the response is truncated and if the
requester supports TCP, it SHOULD try the query again using
TCP.

.........
         
Truncated responses MUST NOT be saved (cached) and later
used in such a way that the fact that they are truncated is
lost.

Instead of following up with a TCP query which is unsupported, currently the library will follow up with DNS A query. Based on the recommendation above, it's better to discard the response and raise error.

Thanks to Daniel Schopf for the report.

[riza]

In 6000:

Close #2200: Ignore/discard DNS SRV response with truncated flag set.