As long as at least one IP config method completes, and as long as
methods that the user required to complete do complete, allow the
connection to complete.
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.
