core: add an UnlockRequired property that provides lock status (bgo #604551)

Clients can check the property to determine lock/unlock status and thus
unlock the modem before trying to connect if required.

Bits of the patch by dcbw (see the bug).

src/mm-modem.c

@@ -694,6 +694,15 @@ mm_modem_init (gpointer g_iface)
                                FALSE,
                                G_PARAM_READABLE));
 
+    g_object_interface_install_property
+        (g_iface,
+         g_param_spec_string (MM_MODEM_UNLOCK_REQUIRED,
+                               "UnlockRequired",
+                               "Whether or not the modem requires an unlock "
+                               "code to become usable, and if so, which unlock code is required",
+                               NULL,
+                               G_PARAM_READABLE));
+
     /* Signals */
     g_signal_new ("state-changed",
                   iface_type,