After the DUN branch merge (I think?) a number of NM_IS_MODEM
calls were left around which now always return FALSE since
NMDeviceCdma and NMDeviceGsm aren't subclasses of NMModem anymore.
But we still need generic "is this a modem subclass" checks in
a few places, so add a modem base class that both the GSM and
CDMA device classes inherit from and use that.
Plus, we want to consolidate a ton of the common code in
nm-device-gsm.c and nm-device-cdma.c into the base class in the
future anyway.
