voice,api: new 'EmergencyOnly' boolean flag

This new flag allows users of the API to know whether general purpose
voice calls are allowed or otherwise only voice calls to the
registered emergency numbers should be performed.

ModemManager won't really do any distinction between emergency and
non-emergency calls at this point, this flag is just an early
indication for the user of the API that no normal voice call should be
attempted.

src/mm-iface-modem-voice.c

@@ -2879,6 +2879,20 @@ mm_iface_modem_voice_initialize_finish (MMIfaceModemVoice *self,
     return g_task_propagate_boolean (G_TASK (res), error);
 }
 
+static gboolean
+modem_state_to_emergency_only (GBinding     *binding,
+                               const GValue *from_value,
+                               GValue       *to_value)
+{
+    MMModemState state;
+
+    /* If the modem is REGISTERED, we allow any kind of call, otherwise
+     * only emergency calls */
+    state = g_value_get_enum (from_value);
+    g_value_set_boolean (to_value, (state < MM_MODEM_STATE_REGISTERED));
+    return TRUE;
+}
+
 void
 mm_iface_modem_voice_initialize (MMIfaceModemVoice *self,
                                  GCancellable *cancellable,
@@ -2899,6 +2913,12 @@ mm_iface_modem_voice_initialize (MMIfaceModemVoice *self,
         g_object_set (self,
                       MM_IFACE_MODEM_VOICE_DBUS_SKELETON, skeleton,
                       NULL);
+
+        g_object_bind_property_full (self, MM_IFACE_MODEM_STATE,
+                                     skeleton, "emergency-only",
+                                     G_BINDING_DEFAULT | G_BINDING_SYNC_CREATE,
+                                     (GBindingTransformFunc) modem_state_to_emergency_only,
+                                     NULL, NULL, NULL);
     }
 
     /* Perform async initialization here */