Only warn on invalid redeclarations of predefined entities

Downgrade the error message to a warning since the error was ignored,
anyway. Also print the name of redeclared entity. For a proper fix that
also shows filename and line number of the invalid redeclaration, we'd
have to

- pass the parser context to the entity functions somehow, or
- make these functions return distinct error codes.

Partial fix for #308.

entities.c

@@ -94,6 +94,23 @@ xmlEntitiesErr(xmlParserErrors code, const char *msg)
     __xmlSimpleError(XML_FROM_TREE, code, NULL, msg, NULL);
 }
 
+/**
+ * xmlEntitiesWarn:
+ * @code:  the error code
+ * @msg:  the message
+ *
+ * Handle an out of memory condition
+ */
+static void LIBXML_ATTR_FORMAT(2,0)
+xmlEntitiesWarn(xmlParserErrors code, const char *msg, const xmlChar *str1)
+{
+    __xmlRaiseError(NULL, NULL, NULL,
+                NULL, NULL, XML_FROM_TREE, code,
+                XML_ERR_WARNING, NULL, 0,
+                (const char *)str1, NULL, NULL, 0, 0,
+                msg, (const char *)str1, NULL);
+}
+
 /*
  * xmlFreeEntity : clean-up an entity record.
  */
@@ -255,9 +272,9 @@ xmlAddEntity(xmlDtdPtr dtd, const xmlChar *name, int type,
                     }
                 }
                 if (!valid) {
-                    xmlEntitiesErr(XML_ERR_ENTITY_PROCESSING,
+                    xmlEntitiesWarn(XML_ERR_ENTITY_PROCESSING,
                             "xmlAddEntity: invalid redeclaration of predefined"
-                            " entity");
+                            " entity '%s'", name);
                     return(NULL);
                 }
             }

result/errors/ent_redecl.xml

@@ -0,0 +1,3 @@
+<?xml version="1.0"?>
+<!DOCTYPE doc>
+<doc/>

result/errors/ent_redecl.xml.ent

@@ -0,0 +1 @@
+warning : xmlAddEntity: invalid redeclaration of predefined entity 'lt'

result/errors/ent_redecl.xml.err

@@ -0,0 +1 @@
+warning : xmlAddEntity: invalid redeclaration of predefined entity 'lt'

result/errors/ent_redecl.xml.str

@@ -0,0 +1 @@
+warning : xmlAddEntity: invalid redeclaration of predefined entity 'lt'

test/errors/ent_redecl.xml

@@ -0,0 +1,4 @@
+<!DOCTYPE doc [
+    <!ENTITY lt '<'>
+]>
+<doc/>