BIND doesn't allow the options section (or any section I'd guess) to be
defined more than once, so whenever you want to set an additional option
you're stuck using weird hacks like this:

services.bind.forwarders = lib.mkForce [ "}; empty-zones-enable no; #" ];

This basically exploits the fact that values coming from the module
options aren't escaped and thus works in a similar vain to how SQL
injection works.

Another option would be to just set configFile to a file that includes
all the options, including zones. That obviously makes the configuration
way less extensible and more awkward to use with the module system.

To make sure this change does work correctly I added a small test just
for that. The test could use some improvements, but better to have a
test rather than none at all. For a future improvement the test could be
merged with the NSD test, because both use the same zone file format.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @peti, @edolstra