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_connect.cc

@@ -21,6 +21,7 @@
 */
 
 #include "mariadb.h"
+#include "mysqld.h"
 #include "sql_priv.h"
 #ifndef __WIN__
 #include <netdb.h>        // getservbyname, servent
@@ -1264,7 +1265,7 @@ void prepare_new_connection_state(THD* thd)
       if (packet_length != packet_error)
         my_error(ER_NEW_ABORTING_CONNECTION, MYF(0),
                  thd->thread_id,
-                 thd->db ? thd->db : "unconnected",
+                 thd->db.str ? thd->db.str : "unconnected",
                  sctx->user ? sctx->user : "unauthenticated",
                  sctx->host_or_ip, "init_connect command failed");
       thd->server_status&= ~SERVER_STATUS_CLEAR_SET;