Introduce netdev API

This module provides several operations on network devices
(interfaces), like monitoring changes, setting addresses, routes, link
state, etc.
It also supports managing network interfaces on several different netns
concurrently.

These functionalitites will be used by the tun module included in a
follow-up patch.

Change-Id: I7a00c0445a89e088676a4897061b65196d9197f1
diff --git a/TODO-RELEASE b/TODO-RELEASE
index f9b31a0..80e6bc1 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -10,3 +10,4 @@
 libosmocore new API osmo_sockaddr_is_any(), osmo_sockaddr_netmask_to_prefixlen()
 libosmocore	ABI breakage		OSMO_NUM_DLIB change affecting internal_cat[]
 libosmocore new API osmo_netns_*()
+libosmocore new API osmo_netdev_*()