data tree CHANGE prepare updated validation flags

Except LYD_OPT_VAL_DATA_ONLY, none are implemented, only defined.
Operations (RPC/action/rpc-reply/notification) cannot be parsed
nor validated as they will have a separate function implemented.
Also, mandatory validation implemented.
diff --git a/src/common.h b/src/common.h
index 1f0cbe7..cc8b82a 100644
--- a/src/common.h
+++ b/src/common.h
@@ -229,6 +229,7 @@
 #define LY_VCODE_DEV_NOT_PRESENT LYVE_REFERENCE, "Invalid deviation %s \"%s\" property \"%s\" which is not present."
 
 #define LY_VCODE_NOWHEN         LYVE_DATA, "When condition \"%s\" not satisfied."
+#define LY_VCODE_NOMAND         LYVE_DATA, "Mandatory node \"%s\" instance does not exist."
 
 /******************************************************************************
  * Context