[rilmodem][WIP] Implementing call waiting query and setting

Bringing call waiting query and setting as descibed in call
settings API to Rilmodem. Since ofono interface does not offer
chance to specify service class in same manner as RIL interface
implementation is hard coded for voice call.

Signed-off-by: jussi.kangas@tieto.com
Merge pull request #18 from jkangas/master

Request for bringing call waiting query and setting to main branch