[9.18] Fix xfrin_connect_done() error paths
The xfrin_connect_done() had several problems:
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it would not add the server to unreachable table in case of the failure coming from the dispatch [GL #3989 (closed)]
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if dns_dispatch_checkperm() disallowed the connection, the xfr would be left undetached
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if xfrin_send_request() failed to send the request, the xfr would be left undetached
All of these have been fixed in this commit.
(cherry picked from commit 536e439c)
Closes #3989 (closed)
Backport of MR!7774