Fixed bug mdev-464.

The value of system variable use_stat_tables was always reset to 0 ('never') 
by mistake at the launch of the server.

mysql-test/r/stat_tables.result

+select @@global.use_stat_tables;
+@@global.use_stat_tables
+COMPLEMENTARY
+select @@session.use_stat_tables;
+@@session.use_stat_tables
+COMPLEMENTARY
 set @save_use_stat_tables=@@use_stat_tables;
 set use_stat_tables='preferably';
 DROP DATABASE IF EXISTS dbt3_s001;

mysql-test/r/stat_tables_innodb.result

 SET SESSION STORAGE_ENGINE='InnoDB';
 set @save_optimizer_switch_for_stat_tables_test=@@optimizer_switch;
 set optimizer_switch='extended_keys=on';
+select @@global.use_stat_tables;
+@@global.use_stat_tables
+COMPLEMENTARY
+select @@session.use_stat_tables;
+@@session.use_stat_tables
+COMPLEMENTARY
 set @save_use_stat_tables=@@use_stat_tables;
 set use_stat_tables='preferably';
 DROP DATABASE IF EXISTS dbt3_s001;

mysql-test/t/stat_tables.test

 --source include/have_stat_tables.inc
 
+select @@global.use_stat_tables;
+select @@session.use_stat_tables;
+
 set @save_use_stat_tables=@@use_stat_tables;
 
 set use_stat_tables='preferably';

sql/mysqld.cc

@@ -3752,8 +3752,6 @@ static int init_common_variables()
   global_system_variables.character_set_results= default_charset_info;
   global_system_variables.character_set_client=  default_charset_info;
 
-  global_system_variables.use_stat_tables= 0;
- 
   if (!(character_set_filesystem=
         get_charset_by_csname(character_set_filesystem_name,
                               MY_CS_PRIMARY, MYF(MY_WME))))