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_queue.h

@@ -31,7 +31,7 @@ extern PSI_cond_key key_COND_queue_state;
 #endif /* HAVE_PSI_INTERFACE */
 
 #include "queues.h"                             // QUEUE
-#include "sql_string.h"                         /* LEX_STRING */
+#include "sql_string.h"                         /* LEX_CSTRING */
 #include "my_time.h"                    /* my_time_t, interval_type */
 
 class Event_basic;
@@ -60,14 +60,14 @@ public:
                bool *created);
 
   void
-  update_event(THD *thd, LEX_STRING dbname, LEX_STRING name,
+  update_event(THD *thd, const LEX_CSTRING *dbname, const LEX_CSTRING *name,
                Event_queue_element *new_element);
 
   void
-  drop_event(THD *thd, LEX_STRING dbname, LEX_STRING name);
+  drop_event(THD *thd, const LEX_CSTRING *dbname, const LEX_CSTRING *name);
 
   void
-  drop_schema_events(THD *thd, LEX_STRING schema);
+  drop_schema_events(THD *thd, const LEX_CSTRING *schema);
 
   void
   recalculate_activation_times(THD *thd);
@@ -98,12 +98,12 @@ private:
             const char *src_func, const char *src_file, uint src_line);
 
   void
-  find_n_remove_event(LEX_STRING db, LEX_STRING name);
+  find_n_remove_event(const LEX_CSTRING *db, const LEX_CSTRING *name);
 
 
   void
-  drop_matching_events(THD *thd, LEX_STRING pattern,
-                       bool (*)(LEX_STRING, Event_basic *));
+  drop_matching_events(THD *thd, const LEX_CSTRING *pattern,
+                       bool (*)(const LEX_CSTRING*, Event_basic *));
 
 
   void