vty: Add reserved nodes to enum node_type

Currently every time a node is added to enum node_type, this
constitutes an ABI change, since _LAST_OSMOVTY_NODE will get
incremented accordingly. In this case, every project that adds new
node type based on that value will have to be recompiled.

This commit adds 4 spare node type values, which can be replaced
one-by-one by new real types until they are exhausted to avoid
this kind of ABI change.

Sponsored-by: On-Waves ehf
diff --git a/include/osmocom/vty/command.h b/include/osmocom/vty/command.h
index 890f0d6..2ef4109 100644
--- a/include/osmocom/vty/command.h
+++ b/include/osmocom/vty/command.h
@@ -84,6 +84,15 @@
 	L_NS_NODE,		/*!< \brief NS node in libosmo-gb. */
 	L_BSSGP_NODE,		/*!< \brief BSSGP node in libosmo-gb. */
 
+	/*
+	 * When adding new nodes to the libosmocore project, these nodes can be
+	 * used to avoid ABI changes for unrelated projects.
+	 */
+	RESERVED1_NODE,		/*!< \brief Reserved for later extensions */
+	RESERVED2_NODE,		/*!< \brief Reserved for later extensions */
+	RESERVED3_NODE,		/*!< \brief Reserved for later extensions */
+	RESERVED4_NODE,		/*!< \brief Reserved for later extensions */
+
 	_LAST_OSMOVTY_NODE
 };