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_insert.cc

@@ -289,7 +289,8 @@ static int check_insert_fields(THD *thd, TABLE_LIST *table_list,
 
     if (check_unique && thd->dup_field)
     {
-      my_error(ER_FIELD_SPECIFIED_TWICE, MYF(0), thd->dup_field->field_name);
+      my_error(ER_FIELD_SPECIFIED_TWICE, MYF(0),
+               thd->dup_field->field_name.str);
       DBUG_RETURN(-1);
     }
   }
@@ -328,7 +329,8 @@ static bool has_no_default_value(THD *thd, Field *field, TABLE_LIST *table_list)
     else
     {
       push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN, ER_NO_DEFAULT_FOR_FIELD,
-                          ER_THD(thd, ER_NO_DEFAULT_FOR_FIELD), field->field_name);
+                          ER_THD(thd, ER_NO_DEFAULT_FOR_FIELD),
+                          field->field_name.str);
     }
     return thd->really_abort_on_warning();
   }