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_handler.cc

@@ -549,7 +549,7 @@ SQL_HANDLER *mysql_ha_find_handler(THD *thd, const char *name)
 
 static bool
 mysql_ha_fix_cond_and_key(SQL_HANDLER *handler, 
-                          enum enum_ha_read_modes mode, char *keyname,
+                          enum enum_ha_read_modes mode, const char *keyname,
                           List<Item> *key_expr,
                           Item *cond, bool in_prepare)
 {
@@ -671,7 +671,7 @@ mysql_ha_fix_cond_and_key(SQL_HANDLER *handler,
 */
  
 bool mysql_ha_read(THD *thd, TABLE_LIST *tables,
-                   enum enum_ha_read_modes mode, char *keyname,
+                   enum enum_ha_read_modes mode, const char *keyname,
                    List<Item> *key_expr,
                    enum ha_rkey_function ha_rkey_mode, Item *cond,
                    ha_rows select_limit_cnt, ha_rows offset_limit_cnt)
@@ -926,7 +926,8 @@ err0:
 */
 
 SQL_HANDLER *mysql_ha_read_prepare(THD *thd, TABLE_LIST *tables,
-                                   enum enum_ha_read_modes mode, char *keyname,
+                                   enum enum_ha_read_modes mode,
+                                   const char *keyname,
                                    List<Item> *key_expr, Item *cond)
 {
   SQL_HANDLER *handler;