this is needed to differentiate route/cfwd

Make this thing readable by wasting a couple of bytes for spaces and newlines.

As an added bonus, save some stack space by making usage_string a static const.
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>

make sure dtmfinband gets initialized.
make gk-identifier and gk-interface settings optional (documentation sayed that about gk-identifier already)