validation NEW if-feature validation

Some refactoring included.
diff --git a/src/common.h b/src/common.h
index 3be741d..57d1808 100644
--- a/src/common.h
+++ b/src/common.h
@@ -235,6 +235,7 @@
 #define LY_VCODE_NOUNIQ         LYVE_DATA, "Unique data leaf(s) \"%s\" not satisfied in \"%s\" and \"%s\"."
 #define LY_VCODE_DUP            LYVE_DATA, "Duplicate instance of \"%s\"."
 #define LY_VCODE_DUPCASE        LYVE_DATA, "Data for both cases \"%s\" and \"%s\" exist."
+#define LY_VCODE_NOIFF          LYVE_DATA, "Data are disabled by \"%s\" schema node if-feature."
 
 /******************************************************************************
  * Context