Fix DiG UDP query retry and fail-over bug

Fix DiG UDP query retry and fail-over bug

When the `udp_ready()` callback function gets called with a failure
result code, DiG erroneously cancels the lookup.

Copy the logic behind `tcp_connected()` callback function into
`udp_ready()` so that DiG will now retry the failed query (if retries
are enabled) and then, if it fails again, it will fail-over to the next
server in the list, which synchronizes the behavior between TCP and UDP
modes.

Also, `udp_ready()` was calling `lookup_detach()` without calling
`lookup_attach()` first, but the issue was masked behind the fact
that `clear_current_lookup()` wasn't being called when needed, and
`lookup_detach()` was compensating for that. This also has been fixed.

(cherry picked from commit 3f3108552577c326b4dab6c3b631c51cf0040144)