Introduce netns API

Write a new API and implementation to manage network namespace related
operations.

This will be used by the upcoming tundev module.

Change-Id: I0f2fba2fa42250a07211a7b7f479498f27c529da
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 86363e1..f9b31a0 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -9,3 +9,4 @@
 #library	what			description / commit summary line
 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_*()