Introduce tundev API

The data structre is held private so that it can be easily extended in
the future.

Change-Id: I6f8324da9ba49b9249682e2ec5b45297f18dd8c2
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 80e6bc1..7a8833d 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -11,3 +11,4 @@
 libosmocore	ABI breakage		OSMO_NUM_DLIB change affecting internal_cat[]
 libosmocore new API osmo_netns_*()
 libosmocore new API osmo_netdev_*()
+libosmocore new API osmo_tundev_*()