VTY: improve VTY prompt and make sure exit/end works everywhere

Some nodes below 'config' didn't have ournode_exit / ournode_end,
and thus were not able to properly perform this function.  exit should
always only go back one level, while end drops us back to ENABLE_NODE.

The prompt now represents the nesting level, and there's one consistent
space after the final prompt character (typically #).
diff --git a/openbsc/src/libbsc/bsc_vty.c b/openbsc/src/libbsc/bsc_vty.c
index d7cdee6..b91304d 100644
--- a/openbsc/src/libbsc/bsc_vty.c
+++ b/openbsc/src/libbsc/bsc_vty.c
@@ -97,25 +97,25 @@
 
 struct cmd_node net_node = {
 	GSMNET_NODE,
-	"%s(network)#",
+	"%s(config-net)# ",
 	1,
 };
 
 struct cmd_node bts_node = {
 	BTS_NODE,
-	"%s(bts)#",
+	"%s(config-net-bts)# ",
 	1,
 };
 
 struct cmd_node trx_node = {
 	TRX_NODE,
-	"%s(trx)#",
+	"%s(config-net-bts-trx)# ",
 	1,
 };
 
 struct cmd_node ts_node = {
 	TS_NODE,
-	"%s(ts)#",
+	"%s(config-net-bts-trx-ts)# ",
 	1,
 };