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/include/osmocom/core/Makefile.am b/include/osmocom/core/Makefile.am
index 1b0a1ce..e88ce89 100644
--- a/include/osmocom/core/Makefile.am
+++ b/include/osmocom/core/Makefile.am
@@ -35,6 +35,7 @@
 	stats.h \
 	macaddr.h \
 	msgb.h \
+	netns.h \
 	panic.h \
 	prbs.h \
 	prim.h \