[rilmodem][WIP] Implementing call forwarding query and erasure

Introducing skeleton implementation for call forwarding in rilmodem
Bringing call forwarding query to rilmodem. Bringing call forwarding
erasure to rilmodem

Signed-off-by: jussi.kangas@tieto.com
[rilmodem][WIP] Adding support for class usage in call settings

Bugfix for previously not handling the service class offered by
ofono core.

Signed-off-by: jussi.kangas@tieto.com