core: fix NMManager in private-bus-only case

NMManager was failing to initialize if there was only a private bus,
despite the fact that this is exactly the use case that the private
bus was added for.

The only other potentially-failing code in nm_manager_new() was adding
prop_filter to the D-Bus connection, but this can't really fail, so
just assert that it doesn't. And now, nm_manager_new() always
succeeds, so update the caller for that.

src/main.c

@@ -459,13 +459,7 @@ main (int argc, char *argv[])
 	                          net_enabled,
 	                          wifi_enabled,
 	                          wwan_enabled,
-	                          wimax_enabled,
-	                          &error);
-	if (manager == NULL) {
-		nm_log_err (LOGD_CORE, "failed to initialize the network manager: %s",
-		            error && error->message ? error->message : "(unknown)");
-		goto done;
-	}
+	                          wimax_enabled);
 
 	g_signal_connect (manager, NM_MANAGER_CONFIGURE_QUIT, G_CALLBACK (manager_configure_quit), config);