Refactor cssn into ofono_ssn

Refactor cssn into ofono_ssn

Make this into a fully fledged entity, with a driver instead of the
current kludge.  This means modem drivers can actually choose whether
to instantiate a CSSN atom or not

Move the notification functions from voicecall.c into ssn.c.

Move the cssn.h header into include/ssn.h and refactor

Update call barring to utilize ofono_ssn and use the new atom_watch
functionality to detect when ssn has been added or removed