[rilmodem] Listen only network status and core state to set attached state

There is no real equivalent for ofono attached state in RIL interface. So
far attached state has been set based on data registration data. This is
something that cannot be controlled directly by ofono. However ofono core
is implemented on idea that state is controlled by it. Because of that
attached state can be based on data registration state only partially.
This change takes only roaming state from data registration otherwise
attached state is based to core state and network state sent it by
network registration.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #77 from jkangas/master

Request for pulling listening only network and ofono core status to set ofono attached to main branch