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/event_data_objects.h

@@ -37,10 +37,10 @@ public:
   ~Event_queue_element_for_exec();
 
   bool
-  init(LEX_STRING dbname, LEX_STRING name);
+  init(LEX_CSTRING dbname, LEX_CSTRING name);
 
-  LEX_STRING dbname;
-  LEX_STRING name;
+  LEX_CSTRING dbname;
+  LEX_CSTRING name;
   bool dropped;
   THD *thd;
 
@@ -58,9 +58,9 @@ protected:
 
 public:
 
-  LEX_STRING dbname;
-  LEX_STRING name;
-  LEX_STRING definer;// combination of user and host
+  LEX_CSTRING dbname;
+  LEX_CSTRING name;
+  LEX_CSTRING definer;// combination of user and host
 
   Time_zone *time_zone;
 
@@ -75,7 +75,7 @@ protected:
   load_string_fields(Field **fields, ...);
 
   bool
-  load_time_zone(THD *thd, const LEX_STRING tz_name);
+  load_time_zone(THD *thd, const LEX_CSTRING *tz_name);
 };
 
 
@@ -122,12 +122,12 @@ class Event_timed : public Event_queue_element
   void operator=(Event_timed &);
 
 public:
-  LEX_STRING body;
+  LEX_CSTRING body;
 
-  LEX_STRING definer_user;
-  LEX_STRING definer_host;
+  LEX_CSTRING definer_user;
+  LEX_CSTRING definer_host;
 
-  LEX_STRING comment;
+  LEX_CSTRING comment;
 
   ulonglong created;
   ulonglong modified;
@@ -135,7 +135,7 @@ public:
   sql_mode_t sql_mode;
 
   class Stored_program_creation_ctx *creation_ctx;
-  LEX_STRING body_utf8;
+  LEX_CSTRING body_utf8;
 
   Event_timed();
   virtual ~Event_timed();
@@ -154,9 +154,9 @@ public:
 class Event_job_data : public Event_basic
 {
 public:
-  LEX_STRING body;
-  LEX_STRING definer_user;
-  LEX_STRING definer_host;
+  LEX_CSTRING body;
+  LEX_CSTRING definer_user;
+  LEX_CSTRING definer_host;
 
   sql_mode_t sql_mode;
 
@@ -182,11 +182,12 @@ private:
 
 /* Compares only the schema part of the identifier */
 bool
-event_basic_db_equal(LEX_STRING db, Event_basic *et);
+event_basic_db_equal(const LEX_CSTRING *db, Event_basic *et);
 
 /* Compares the whole identifier*/
 bool
-event_basic_identifier_equal(LEX_STRING db, LEX_STRING name, Event_basic *b);
+event_basic_identifier_equal(const LEX_CSTRING *db, const LEX_CSTRING *name,
+                             Event_basic *b);
 
 /**
   @} (End of group Event_Scheduler)