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/libmgcp/mgcp_vty.c b/openbsc/src/libmgcp/mgcp_vty.c
index 833908a..9421f4f 100644
--- a/openbsc/src/libmgcp/mgcp_vty.c
+++ b/openbsc/src/libmgcp/mgcp_vty.c
@@ -786,9 +786,7 @@
 	install_element(CONFIG_NODE, &cfg_mgcp_cmd);
 	install_node(&mgcp_node, config_write_mgcp);
 
-	install_default(MGCP_NODE);
-	install_element(MGCP_NODE, &ournode_exit_cmd);
-	install_element(MGCP_NODE, &ournode_end_cmd);
+	bsc_install_default(MGCP_NODE);
 	install_element(MGCP_NODE, &cfg_mgcp_local_ip_cmd);
 	install_element(MGCP_NODE, &cfg_mgcp_bts_ip_cmd);
 	install_element(MGCP_NODE, &cfg_mgcp_bind_ip_cmd);
@@ -820,9 +818,7 @@
 
 	install_element(MGCP_NODE, &cfg_mgcp_trunk_cmd);
 	install_node(&trunk_node, config_write_trunk);
-	install_default(TRUNK_NODE);
-	install_element(TRUNK_NODE, &ournode_exit_cmd);
-	install_element(TRUNK_NODE, &ournode_end_cmd);
+	bsc_install_default(TRUNK_NODE);
 	install_element(TRUNK_NODE, &cfg_trunk_payload_number_cmd);
 	install_element(TRUNK_NODE, &cfg_trunk_payload_name_cmd);
 	install_element(TRUNK_NODE, &cfg_trunk_payload_number_cmd_old);