+/**
+ * @brief Initialize a feature graph arc
+ * @param vm vlib main structure pointer
+ * @param vcm vnet config main structure pointer
+ * @param feature_start_nodes names of start-nodes which use this
+ * feature graph arc
+ * @param num_feature_start_nodes number of start-nodes
+ * @param first_reg first element in
+ * [an __attribute__((constructor)) function built, or
+ * otherwise created] singly-linked list of feature registrations
+ * @param [out] in_feature_nodes returned vector of
+ * topologically-sorted feature node names, for use in
+ * show commands
+ * @returns 0 on success, otherwise an error message. Errors
+ * are fatal since they invariably involve mistyped node-names, or
+ * genuinely missing node-names
+ */