This command will give us URCs whenever the extended list of current
calls changes, which includes information about the actual state of
each call, even for calls in waiting state.
Therefore, as this is a URC that applies to all calls, it's enabled
and disabled as part of the modem voice interface, instead of doing it
as part of the call object itself (i.e. not treated as an in-call URC).
Implement a new interface to keep the code shared between the QMI and
non-QMI modem implementations.
While doing that, also fix the parent interface pointer handling, so
that it isn't a static pointer applicable to all modems, and make it a
per-modem specific pointer. Without this fix, ModemManager would crash
if e.g. running with both a QMI and non-QMI Cinterion modem at the
same time.
The new shared Cinterion logic will be in charge of managing all GPS
sources not already managed by the parent interface. E.g. if the
parent implementation already supports QMI-based GPS location (using
the LOC service for example) prefer that to the custom AT-based
logic.
