dictionary REFACTOR unify functions pattern to return LY_ERR

Change API functions manipulating with libyang dictionary to return
LY_ERR value as a primary indicator of the result and the value from the
dictionary is returned via optional output parameter.
diff --git a/src/tree_data_free.c b/src/tree_data_free.c
index 176d39f..333c5d6 100644
--- a/src/tree_data_free.c
+++ b/src/tree_data_free.c
@@ -77,7 +77,7 @@
 }
 
 static void
-ly_free_attr(const struct ly_ctx *ctx, struct lyd_attr *attr, ly_bool siblings)
+lyd_free_attr(const struct ly_ctx *ctx, struct lyd_attr *attr, ly_bool siblings)
 {
     struct lyd_attr *iter;
     LY_ARRAY_COUNT_TYPE u;
@@ -128,15 +128,15 @@
 }
 
 API void
-ly_free_attr_single(const struct ly_ctx *ctx, struct lyd_attr *attr)
+lyd_free_attr_single(const struct ly_ctx *ctx, struct lyd_attr *attr)
 {
-    ly_free_attr(ctx, attr, 0);
+    lyd_free_attr(ctx, attr, 0);
 }
 
 API void
-ly_free_attr_siblings(const struct ly_ctx *ctx, struct lyd_attr *attr)
+lyd_free_attr_siblings(const struct ly_ctx *ctx, struct lyd_attr *attr)
 {
-    ly_free_attr(ctx, attr, 1);
+    lyd_free_attr(ctx, attr, 1);
 }
 
 void
@@ -197,7 +197,7 @@
     }
 
     if (!node->schema) {
-        ly_free_attr_siblings(LYD_CTX(node), opaq->attr);
+        lyd_free_attr_siblings(LYD_CTX(node), opaq->attr);
     } else {
         /* free the node's metadata */
         lyd_free_meta_siblings(node->meta);