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_expression_cache.cc

@@ -97,6 +97,7 @@ void Expression_cache_tmptable::init()
   List_iterator<Item> li(items);
   Item_iterator_list it(li);
   uint field_counter;
+  LEX_CSTRING cache_table_name= { STRING_WITH_LEN("subquery-cache-table") };
   DBUG_ENTER("Expression_cache_tmptable::init");
   DBUG_ASSERT(!inited);
   inited= TRUE;
@@ -124,7 +125,7 @@ void Expression_cache_tmptable::init()
                                         TMP_TABLE_ALL_COLUMNS) &
                                         ~TMP_TABLE_FORCE_MYISAM),
                                       HA_POS_ERROR,
-                                      (char *)"subquery-cache-table",
+                                      &cache_table_name,
                                       TRUE)))
   {
     DBUG_PRINT("error", ("create_tmp_table failed, caching switched off"));