Changed database, tablename and alias to be LEX_CSTRING

This was done in, among other things:
- thd->db and thd->db_length
- TABLE_LIST tablename, db, alias and schema_name
- Audit plugin database name
- lex->db
- All db and table names in Alter_table_ctx
- st_select_lex db

Other things:
- Changed a lot of functions to take const LEX_CSTRING* as argument
  for db, table_name and alias. See init_one_table() as an example.
- Changed some function arguments from LEX_CSTRING to const LEX_CSTRING
- Changed some lists from LEX_STRING to LEX_CSTRING
- threads_mysql.result changed because process list_db wasn't always
  correctly updated
- New append_identifier() function that takes LEX_CSTRING* as arguments
- Added new element tmp_buff to Alter_table_ctx to separate temp name
  handling from temporary space
- Ensure we store the length after my_casedn_str() of table/db names
- Removed not used version of rename_table_in_stat_tables()
- Changed Natural_join_column::table_name and db_name() to never return
  NULL (used for print)
- thd->get_db() now returns db as a printable string (thd->db.str or "")

sql/sql_select.h

@@ -2377,7 +2377,7 @@ int append_possible_keys(MEM_ROOT *alloc, String_list &list, TABLE *table,
 TABLE *create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
 			ORDER *group, bool distinct, bool save_sum_fields,
 			ulonglong select_options, ha_rows rows_limit,
-			const char* alias, bool do_not_open=FALSE,
+                        const LEX_CSTRING *alias, bool do_not_open=FALSE,
                         bool keep_row_order= FALSE);
 void free_tmp_table(THD *thd, TABLE *entry);
 bool create_internal_tmp_table_from_heap(THD *thd, TABLE *table,