[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>
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
[rilmodem] Fix crash when going offline
Merge pull request #88 from marttipiirainen/gprs_crash

[rilmodem] Fix crash when going offline
[rilmodem] Enable context deactivation when RoamingAllowed is set to FALSE

Without this change context is left active if only RoamingAllowed is called
in ConnMan API when there is active context. Connection cannot be used
but it is misleading from user point of view and also agains ofono spec.
Therefore it is better enable deactivation.

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