vty: Use generic 'end' and 'exit' commands

Add bsc_install_default() and replace all install_default()

This patch adds bsc_install_default() which calls install_default()
and add 'exit' and 'end'. All other calls to install_default() are
replaced by calls to bsc_install_default().

Since 'exit' and 'end' are now added automatically to each node, the
explicit registrations of these commands are removed by this patch,
too.

The related tests succeed now without work-arounds (except for the
'config' node itself which is part of libosmocore).
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
index b2ec156..9eab574 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_vty.c
@@ -1192,9 +1192,7 @@
 	/* nat group */
 	install_element(CONFIG_NODE, &cfg_nat_cmd);
 	install_node(&nat_node, config_write_nat);
-	install_default(NAT_NODE);
-	install_element(NAT_NODE, &ournode_exit_cmd);
-	install_element(NAT_NODE, &ournode_end_cmd);
+	bsc_install_default(NAT_NODE);
 	install_element(NAT_NODE, &cfg_nat_msc_ip_cmd);
 	install_element(NAT_NODE, &cfg_nat_msc_port_cmd);
 	install_element(NAT_NODE, &cfg_nat_auth_time_cmd);
@@ -1235,18 +1233,14 @@
 	install_element(NAT_NODE, &cfg_nat_pgroup_cmd);
 	install_element(NAT_NODE, &cfg_nat_no_pgroup_cmd);
 	install_node(&pgroup_node, config_write_pgroup);
-	install_default(PGROUP_NODE);
-	install_element(PGROUP_NODE, &ournode_exit_cmd);
-	install_element(PGROUP_NODE, &ournode_end_cmd);
+	bsc_install_default(PGROUP_NODE);
 	install_element(PGROUP_NODE, &cfg_pgroup_lac_cmd);
 	install_element(PGROUP_NODE, &cfg_pgroup_no_lac_cmd);
 
 	/* BSC subgroups */
 	install_element(NAT_NODE, &cfg_bsc_cmd);
 	install_node(&bsc_node, config_write_bsc);
-	install_default(NAT_BSC_NODE);
-	install_element(NAT_BSC_NODE, &ournode_exit_cmd);
-	install_element(NAT_BSC_NODE, &ournode_end_cmd);
+	bsc_install_default(NAT_BSC_NODE);
 	install_element(NAT_BSC_NODE, &cfg_bsc_token_cmd);
 	install_element(NAT_BSC_NODE, &cfg_bsc_lac_cmd);
 	install_element(NAT_BSC_NODE, &cfg_bsc_no_lac_cmd);