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_plugin.h

@@ -90,7 +90,7 @@ struct st_ptr_backup {
 
 struct st_plugin_dl
 {
-  LEX_STRING dl;
+  LEX_CSTRING dl;
   void *handle;
   struct st_maria_plugin *plugins;
   st_ptr_backup *ptr_backup;
@@ -105,7 +105,7 @@ struct st_plugin_dl
 
 struct st_plugin_int
 {
-  LEX_STRING name;
+  LEX_CSTRING name;
   struct st_maria_plugin *plugin;
   struct st_plugin_dl *plugin_dl;
   st_ptr_backup *ptr_backup;
@@ -163,18 +163,18 @@ extern const char *plugin_maturity_names[];
 extern int plugin_init(int *argc, char **argv, int init_flags);
 extern void plugin_shutdown(void);
 void add_plugin_options(DYNAMIC_ARRAY *options, MEM_ROOT *mem_root);
-extern bool plugin_is_ready(const LEX_STRING *name, int type);
+extern bool plugin_is_ready(const LEX_CSTRING *name, int type);
 #define my_plugin_lock_by_name(A,B,C) plugin_lock_by_name(A,B,C)
 #define my_plugin_lock(A,B) plugin_lock(A,B)
 extern plugin_ref plugin_lock(THD *thd, plugin_ref ptr);
-extern plugin_ref plugin_lock_by_name(THD *thd, const LEX_STRING *name,
+extern plugin_ref plugin_lock_by_name(THD *thd, const LEX_CSTRING *name,
                                       int type);
 extern void plugin_unlock(THD *thd, plugin_ref plugin);
 extern void plugin_unlock_list(THD *thd, plugin_ref *list, uint count);
-extern bool mysql_install_plugin(THD *thd, const LEX_STRING *name,
-                                 const LEX_STRING *dl);
-extern bool mysql_uninstall_plugin(THD *thd, const LEX_STRING *name,
-                                   const LEX_STRING *dl);
+extern bool mysql_install_plugin(THD *thd, const LEX_CSTRING *name,
+                                 const LEX_CSTRING *dl);
+extern bool mysql_uninstall_plugin(THD *thd, const LEX_CSTRING *name,
+                                   const LEX_CSTRING *dl);
 extern bool plugin_register_builtin(struct st_mysql_plugin *plugin);
 extern void plugin_thdvar_init(THD *thd);
 extern void plugin_thdvar_cleanup(THD *thd);
@@ -192,7 +192,7 @@ extern bool plugin_foreach_with_mask(THD *thd, plugin_foreach_func *func,
                                      int type, uint state_mask, void *arg);
 extern void sync_dynamic_session_variables(THD* thd, bool global_lock);
 
-extern bool plugin_dl_foreach(THD *thd, const LEX_STRING *dl,
+extern bool plugin_dl_foreach(THD *thd, const LEX_CSTRING *dl,
                               plugin_foreach_func *func, void *arg);
 
 sys_var *find_sys_var_ex(THD *thd, const char *str, size_t length,