Only use forwarders if any are defined

The `named.conf.options` templates defaults to a `forwarders` value of `0.0.0.0`, with a comment asking the user to change it. The `0.0.0.0` setting seems to slow lookups down (could just be coincidence, but if I remove the `forwarders` block altogether things seem to speed up noticeably. 

These patches cause `bind_config_master_forwarders` to default to an empty list, and makes the `named.conf.options` template only include a forwarders block if that list is populated.