gril: Handle DATA_CALL_LIST replies

Change code that was handling UNSOL_DATA_CALL_LIST_CHANGED to also
handle also the replies to RIL_REQUEST_DATA_CALL_LIST requests, as the
format of the returned data is the same.
rilmodem: Deactivate active data calls on start

Active data calls can be around due to previous ofono aborts. Before
this change, this was handled deactivating all potential active calls.
With this change, we ask for the list of active calls so we avoid
sending unneeded messages for non-existing calls.
gril: Allow NULL socket paths for testing
mtkmodem: Add macros to be used for function calls
unit: Add unit tests for MTK specific parcels
build: Add unit tests for MTK parcels
Merge pull request #73 from alfonsosanchezbeato/mtk-followup

MTK followup