MDEV-4928 Merge collation customization improvements

Merging the following MySQL-5.6 changes:
- WL#5624: Collation customization improvements
  http://dev.mysql.com/worklog/task/?id=5624

- WL#4013: Unicode german2 collation
  http://dev.mysql.com/worklog/task/?id=4013

- Bug#62429 XML: ExtractValue, UpdateXML max arg length 127 chars
  http://bugs.mysql.com/bug.php?id=62429
  (required by WL#5624)

include/my_xml.h

@@ -52,8 +52,15 @@ typedef struct xml_stack_st
   int flags;
   enum my_xml_node_type current_node_type;
   char errstr[128];
-  char attr[128];
-  char *attrend;
+
+  struct {
+    char static_buffer[128];
+    char *buffer;
+    size_t buffer_size;
+    char *start;
+    char *end;
+  } attr;
+
   const char *beg;
   const char *cur;
   const char *end;