Don't enable timeouts in dns_dispatch for incoming transfers

Don't enable timeouts in dns_dispatch for incoming transfers

The dns_dispatch_add() call in the dns_xfrin unit had hardcoded 30
second limit.  This meant that any incoming transfer would be stopped in
it didn't finish within 30 seconds limit.  Additionally, dns_xfrin
callback was ignoring the return value from dns_dispatch_getnext() when
restarting the reading from the TCP stream; this could cause transfers
to get stuck waiting for a callback that would never come due to the
dns_dispatch having already been shut down.

Call the dns_dispatch_add() without a timeout and properly handle the
result code from the dns_dispatch_getnext().