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/osmo_bsc_vty.c b/openbsc/src/osmo-bsc/osmo_bsc_vty.c
index f6cf1a0..90b0a0c 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_vty.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_vty.c
@@ -609,7 +609,7 @@
 	install_element(CONFIG_NODE, &cfg_net_bsc_cmd);
 
 	install_node(&bsc_node, config_write_bsc);
-	install_default(BSC_NODE);
+	bsc_install_default(BSC_NODE);
 	install_element(BSC_NODE, &cfg_net_bsc_mid_call_text_cmd);
 	install_element(BSC_NODE, &cfg_net_bsc_mid_call_timeout_cmd);
 	install_element(BSC_NODE, &cfg_net_rf_socket_cmd);
@@ -617,7 +617,7 @@
 	install_element(BSC_NODE, &cfg_net_no_rf_off_time_cmd);
 
 	install_node(&msc_node, config_write_msc);
-	install_default(MSC_NODE);
+	bsc_install_default(MSC_NODE);
 	install_element(MSC_NODE, &cfg_net_bsc_token_cmd);
 	install_element(MSC_NODE, &cfg_net_bsc_ncc_cmd);
 	install_element(MSC_NODE, &cfg_net_bsc_mcc_cmd);