Instead of a bizare mechanism of signals back to the manager
object that used to be required because of the user/system settings
split, let each place that needs secrets request those secrets
itself. This flattens the secrets request process a ton and
the code flow significantly.
Previously the get secrets flow was something like this:
nm_act_request_get_secrets ()
nm_secrets_provider_interface_get_secrets ()
emits manager-get-secrets signal
provider_get_secerts ()
system_get_secrets ()
system_get_secrets_idle_cb ()
nm_sysconfig_connection_get_secrets ()
system_get_secrets_reply_cb ()
nm_secrets_provider_interface_get_secrets_result ()
signal failure or success
now instead we do something like this:
nm_agent_manager_get_secrets ()
nm_agent_manager_get_secrets ()
request_start_secrets ()
nm_sysconfig_connection_get_secrets ()
return failure or success to callback
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.
