ggsn: cmdline: no need to specify interface anymore

--gtpnl is now gone, instead you have --gtpkernel that behaves as an on/off
toggle. We full rely on the kernel routing base to select the real device to
transmit.

I have updated ggsn/cmdline.ggo and then run 'gengetopt' to refresh the
automatic code generation for command line options that openggsn uses.
diff --git a/ggsn/gtp-kernel.c b/ggsn/gtp-kernel.c
index 9b298ef..93dea23 100644
--- a/ggsn/gtp-kernel.c
+++ b/ggsn/gtp-kernel.c
@@ -94,7 +94,7 @@
 		    struct in_addr *mask,
 		    struct gengetopt_args_info *args_info)
 {
-	if (!args_info->gtpnl_given)
+	if (!args_info->gtp_linux_given)
 		return 0;
 
 	if (gtp_dev_create(-1, GTP_DEVNAME, gsn->fd0, gsn->fd1u) < 0) {