named-checkconf  doesn't complain about

        sit-secret "zyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcbazyxwvutsrqponmlkjihgfedcba";

But named will fail to start after does all configurations and after the 
"command channel listening". It fails with:

26-Aug-2014 13:45:04.123 load_configuration: bad hex encoding
26-Aug-2014 13:45:04.123 loading configuration: bad hex encoding
26-Aug-2014 13:45:04.123 exiting (due to fatal error)

The ARM grammar says "secret_string" but that is not defined book-wide, 
but for key it is a base-64 encoded string.

For this ticket:

- document what it is restricted too.

- have parser fail about it earlier and specifically say is about 
sit-secret (and so named-checkconf should complain too)

- add test cases for sit-secret  (I saw it wasn't tested so that is what 
I started to do but then noticed other issues)