[rilmodem] numeric value can be null in response to RIL_REQUEST_OPERATOR

According to ril.h numeric value can be null in response to
RIL_REQUEST_OPERATOR. That means phone is unregistered and
current operator should return error

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Fix receiving USSD messages not part default GSM character set

RIL delivers USSD string already decoded in UTF-8 format. Hence there is
no need for extra encoding/decoding in rilmodem driver and ofono core.
This implementation will bypass the encoding/decoding mechanism by
using hexadecimal value 0xFF as data coding scheme (reserved SMS and CBS
data coding scheme value in 3GPP TS 23.038)

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
[voicecall] Shorten response time when playing single DTMF tone

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>
[rilmodem] Implementation for call forwarding activation and deactivation

User can try activate or deactivate the call forwarding from keyboard
instead of registration or erasure. Implementing lacking messages

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Fixed memory leak in provision_get_settings
[rilmodem] check that received numeric value is valid

Existing numeric value with invalid content seems to be able to
cause dbus crash. This fix prevents the sending if the value does
not match with ril specification.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>