ncm-resolver: Check all servers listed in configuration

Continue if at least one works, abort if none work.

Fixes #147.
At line 90 you are setting `$testserver` for the else case where `$inf->{dnscache}` is not true, but we don't use the variable anymore.  I suggest getting rid of the `$testserver` scalar and set-up a `@testservers` array instead.  Line 87 can add a single item to the array, line 90 can copy the `@servers` array into it, then lines 109-113 can be replaced with the much simpler:

```
$check = $self->check_dns_servers($host, @testservers);
```

i.e. as no conditional logic would then be required.  I think that would be cleaner.