[rilmodem] Add support for ReleaseAndAnswer()

Add support for ReleaseAndAnswer() which is needed for BT headset that
sends AT+CHLD=1 to bluetoothd. Currently telepathy-ring is not using this
feature.
Tested with release-and-answer test script.

Signed-off-by: Petri Takalokastari <petri.takalokastari@oss.tieto.com>
[rilmodem] Enable roaming allowed setting manipulation

Ofono core listens both network registration and data registration
If context is allowed once in roaming situation network registration
indication updates the status back to attached state. This
change forces the ofono stay in deattached state until ordered
otherwise.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Merge pull request #86 from ptakalok/master

[rilmodem] Add support for ReleaseAndAnswer()
Merge pull request #87 from jkangas/master

Request for pulling roamingallowed support to main branch