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/cmdline.ggo b/ggsn/cmdline.ggo
index 47ff102..7426707 100644
--- a/ggsn/cmdline.ggo
+++ b/ggsn/cmdline.ggo
@@ -34,5 +34,5 @@
 option  "logfile"     - "Logfile for errors"            string no
 option  "loglevel"    - "Global log ldevel"		string default="error" no
 
-option  "gtpnl"       g "GTP kernel support"            string default="eth0" no
+option  "gtp-linux"   g "GTP linux kernel support"      flag	off