Changing field::field_name and Item::name to LEX_CSTRING

Benefits of this patch:
- Removed a lot of calls to strlen(), especially for field_string
- Strings generated by parser are now const strings, less chance of
  accidently changing a string
- Removed a lot of calls with LEX_STRING as parameter (changed to pointer)
- More uniform code
- Item::name_length was not kept up to date. Now fixed
- Several bugs found and fixed (Access to null pointers,
  access of freed memory, wrong arguments to printf like functions)
- Removed a lot of casts from (const char*) to (char*)

Changes:
- This caused some ABI changes
  - lex_string_set now uses LEX_CSTRING
  - Some fucntions are now taking const char* instead of char*
- Create_field::change and after changed to LEX_CSTRING
- handler::connect_string, comment and engine_name() changed to LEX_CSTRING
- Checked printf() related calls to find bugs. Found and fixed several
  errors in old code.
- A lot of changes from LEX_STRING to LEX_CSTRING, especially related to
  parsing and events.
- Some changes from LEX_STRING and LEX_STRING & to LEX_CSTRING*
- Some changes for char* to const char*
- Added printf argument checking for my_snprintf()
- Introduced null_clex_str, star_clex_string, temp_lex_str to simplify
  code
- Added item_empty_name and item_used_name to be able to distingush between
  items that was given an empty name and items that was not given a name
  This is used in sql_yacc.yy to know when to give an item a name.
- select table_name."*' is not anymore same as table_name.*
- removed not used function Item::rename()
- Added comparision of item->name_length before some calls to
  my_strcasecmp() to speed up comparison
- Moved Item_sp_variable::make_field() from item.h to item.cc
- Some minimal code changes to avoid copying to const char *
- Fixed wrong error message in wsrep_mysql_parse()
- Fixed wrong code in find_field_in_natural_join() where real_item() was
  set when it shouldn't
- ER_ERROR_ON_RENAME was used with extra arguments.
- Removed some (wrong) ER_OUTOFMEMORY, as alloc_root will already
  give the error.

TODO:
- Check possible unsafe casts in plugin/auth_examples/qa_auth_interface.c
- Change code to not modify LEX_CSTRING for database name
  (as part of lower_case_table_names)

sql/sql_window.h

@@ -96,7 +96,7 @@ class Window_spec : public Sql_alloc
  public:
   virtual ~Window_spec() {}
 
-  LEX_STRING *window_ref;
+  LEX_CSTRING *window_ref;
 
   SQL_I_List<ORDER> *partition_list;
 
@@ -106,7 +106,7 @@ class Window_spec : public Sql_alloc
 
   Window_spec *referenced_win_spec;
 
-  Window_spec(LEX_STRING *win_ref, 
+  Window_spec(LEX_CSTRING *win_ref, 
               SQL_I_List<ORDER> *part_list,
               SQL_I_List<ORDER> *ord_list,
               Window_frame *win_frame)
@@ -114,11 +114,14 @@ class Window_spec : public Sql_alloc
       partition_list(part_list), order_list(ord_list),
       window_frame(win_frame), referenced_win_spec(NULL) {}
 
-  virtual char *name() { return NULL; }
+  virtual const char *name() { return NULL; }
 
   bool check_window_names(List_iterator_fast<Window_spec> &it);
 
-  char *window_reference() { return window_ref ? window_ref->str : NULL; }
+  const char *window_reference()
+  {
+    return window_ref ? window_ref->str : NULL;
+  }
 
   void join_partition_and_order_lists()
   {
@@ -138,17 +141,17 @@ class Window_def : public Window_spec
 {
  public:
 
-  LEX_STRING *window_name;
+  LEX_CSTRING *window_name;
 
-  Window_def(LEX_STRING *win_name,
-             LEX_STRING *win_ref, 
+  Window_def(LEX_CSTRING *win_name,
+             LEX_CSTRING *win_ref, 
              SQL_I_List<ORDER> *part_list,
              SQL_I_List<ORDER> *ord_list,
              Window_frame *win_frame) 
     : Window_spec(win_ref, part_list, ord_list, win_frame),
       window_name(win_name) {}
  
-  char *name() { return window_name->str; }
+  const char *name() { return window_name->str; }
 
 };