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/parser_xml.c b/src/parser_xml.c
index 496e9ff..43f6925 100644
--- a/src/parser_xml.c
+++ b/src/parser_xml.c
@@ -195,7 +195,7 @@
 
 cleanup:
     if (ret) {
-        ly_free_attr_siblings(xmlctx->ctx, *attr);
+        lyd_free_attr_siblings(xmlctx->ctx, *attr);
         *attr = NULL;
     }
     return ret;
@@ -611,7 +611,7 @@
 
 error:
     lyd_free_meta_siblings(meta);
-    ly_free_attr_siblings(ctx, attr);
+    lyd_free_attr_siblings(ctx, attr);
     lyd_free_tree(node);
     return ret;
 }
@@ -710,7 +710,7 @@
     attr = NULL;
 
 cleanup:
-    ly_free_attr_siblings(xmlctx->ctx, attr);
+    lyd_free_attr_siblings(xmlctx->ctx, attr);
     return ret;
 }
 
@@ -897,7 +897,7 @@
 cleanup:
     if (ret) {
         lyd_free_tree(*envp);
-        ly_free_attr_siblings(xmlctx->ctx, attr);
+        lyd_free_attr_siblings(xmlctx->ctx, attr);
     }
     return ret;
 }