[VTY] Introduce "end" command that works from any level in config

Using "end" you can always return to the "enable" level, and from
there the "show" commands are available.  So no more need for
exit/exit/exit/exit/disable.
diff --git a/openbsc/src/gprs/gb_proxy_vty.c b/openbsc/src/gprs/gb_proxy_vty.c
index babb089..a8501d1 100644
--- a/openbsc/src/gprs/gb_proxy_vty.c
+++ b/openbsc/src/gprs/gb_proxy_vty.c
@@ -159,6 +159,7 @@
 	install_node(&gbproxy_node, config_write_gbproxy);
 	install_default(GBPROXY_NODE);
 	install_element(GBPROXY_NODE, &ournode_exit_cmd);
+	install_element(GBPROXY_NODE, &ournode_end_cmd);
 	install_element(GBPROXY_NODE, &cfg_nsip_bss_local_ip_cmd);
 	install_element(GBPROXY_NODE, &cfg_nsip_bss_local_port_cmd);
 	install_element(GBPROXY_NODE, &cfg_nsip_sgsn_ip_cmd);