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/libmgcp/mgcp_vty.c b/openbsc/src/libmgcp/mgcp_vty.c
index 016ad14..314faa8 100644
--- a/openbsc/src/libmgcp/mgcp_vty.c
+++ b/openbsc/src/libmgcp/mgcp_vty.c
@@ -51,13 +51,13 @@
  */
 struct cmd_node mgcp_node = {
 	MGCP_NODE,
-	"%s(mgcp)#",
+	"%s(config-mgcp)# ",
 	1,
 };
 
 struct cmd_node trunk_node = {
 	TRUNK_NODE,
-	"%s(trunk)#",
+	"%s(config-mgcp-trunk)# ",
 	1,
 };