Some modems are able to handle some proactive commands in their
firmware or otherwise, if the command doesn't require input from user.
Nevertheles ofono may need to update internal state or notify the user
where necessary. With this api the driver can notify core that a
command is being executed in the modem or that a command is finished
executing and the TERMINAL RESPONSE has been sent to SIM. It would
also be possible for a driver to handle a command.