DNS: Fall back to permissive mode if recursor mode is unavailable
The tests when assigning DNS name servers has been extended so that
if no working forwarder can be found, we will test if the local recursor
mode is an option.
If not, we will configure unbound's validator module into permissive
mode so that at least some DNS functionality is available.
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>