AnyEvent::DNS doesn't try other servers on SERVFAIL

[Henrik Pauli]

Hi,

With one of the default servers (80.80.81.81) occasionally misbehaving, we
saw that AnyEvent::DNS only retries on timeouts, but not on errors that
could still result in a different response when asking other nameservers in
the list.  Such as SERVFAIL or REFUSED.

Maybe an elsif around line 1285 in _exec, something like:

```
} elsif ($res->{rc} eq "servfail" || $res->{rc} eq "refused") {
   # server error, try next nameserver
   &$do_retry;
} else {
   # success
```

Though I'm not sure if this will break anything terribly.
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