* Document fallthrough and fix *reverse*
While documenting the fallthrough behavior and testing it I noticed
the did not properly work. This PR does a tiny bit too much as it
- Documents fallthrough
- Fixes fallthrough in reverse
- Makes directives_generate complain on duplicate priorities
- Moved reverse *before* file in middleware.cfg
- Add a test that tests the reverse fallthrough behavior with a file
backend
Fixes#515
* ....and fix the tests
* middleware/reverse: random updates
Make the documentation somewhat shorter (and hopefully clearer in the
process). Also to be on-par with the *auto* middleware, start counting
the referenced zones from 1 (instead of 0).
Some variable cleanups and use the NextOrFailure in the ServeDNS
function.
* More TODOs
