Move kernel GTP support from ggsn/ to lib/

This way, the IP address / route handling between TUN devices and kernel
GTP can be shared, which will provide not only a unified codebase but
also a more consistent behavior.

This also paves the road for to use kernel GTP from sgsnemu in the future.

Related: OS#3214
Change-Id: Ic53a971136edd0d8871fbd6746d7b0090ce3a188
diff --git a/sgsnemu/sgsnemu.c b/sgsnemu/sgsnemu.c
index 92abc70..1e61a09 100644
--- a/sgsnemu/sgsnemu.c
+++ b/sgsnemu/sgsnemu.c
@@ -1571,7 +1571,7 @@
 	if (options.createif) {
 		printf("Setting up interface\n");
 		/* Create a tunnel interface */
-		if (tun_new((struct tun_t **)&tun, options.tun_dev_name)) {
+		if (tun_new((struct tun_t **)&tun, options.tun_dev_name, false, -1, -1)) {
 			SYS_ERR(DSGSN, LOGL_ERROR, 0,
 				"Failed to create tun");
 			exit(1);