DiG: use the same retry and fail-over logic for different failure types

DiG: use the same retry and fail-over logic for different failure types

DiG implements different logic in the `recv_done()` callback function
when processing a failure:

1. For a timed-out query it applies the "retries" logic first, then,
   when it fails, fail-overs to the next server.

2. For an EOF (end-of-file, or unexpected disconnect) error it tries to
   make a single retry attempt (even if the user has requested more
   retries), then, when it fails, fail-overs to the next server.

3. For other types of failures, DiG does not apply the "retries" logic,
   and tries to fail-over to the next servers (again, even if the user
   has requested to make retries).

Simplify the logic and apply the same logic (1) of first retries, and
then fail-over, for different types of failures in `recv_done()`.