MDEV-23554 Wrong default value for foreign_key_checks variable

Sys_var_bit::session_save_default() ignored reverse_semantics property.

mysql-test/suite/innodb/r/foreign_key.result

@@ -167,7 +167,6 @@ PRIMARY KEY  (store_id),
 UNIQUE KEY idx_unique_manager (manager_staff_id),
 CONSTRAINT fk_store_staff FOREIGN KEY (manager_staff_id) REFERENCES staff (staff_id) ON DELETE RESTRICT ON UPDATE CASCADE
 ) ENGINE=InnoDB;
-SET FOREIGN_KEY_CHECKS=DEFAULT;
 LOCK TABLE staff WRITE;
 UNLOCK TABLES;
 DROP TABLES staff, store;

mysql-test/suite/innodb/t/foreign_key.test

@@ -136,7 +136,6 @@ CREATE TABLE store (
   UNIQUE KEY idx_unique_manager (manager_staff_id),
   CONSTRAINT fk_store_staff FOREIGN KEY (manager_staff_id) REFERENCES staff (staff_id) ON DELETE RESTRICT ON UPDATE CASCADE
 ) ENGINE=InnoDB;
-SET FOREIGN_KEY_CHECKS=DEFAULT;
 
 LOCK TABLE staff WRITE;
 UNLOCK TABLES;

mysql-test/suite/sys_vars/r/foreign_key_checks_basic.result

@@ -7,7 +7,7 @@ SET @@session.foreign_key_checks = 1;
 SET @@session.foreign_key_checks = DEFAULT;
 SELECT @@session.foreign_key_checks;
 @@session.foreign_key_checks
-0
+1
 '#---------------------FN_DYNVARS_032_02-------------------------#'
 SET foreign_key_checks = 1;
 SELECT @@foreign_key_checks;

mysql-test/suite/sys_vars/r/unique_checks_basic.result

@@ -7,7 +7,7 @@ SET @@session.unique_checks= 1;
 SET @@session.unique_checks= DEFAULT;
 SELECT @@session.unique_checks;
 @@session.unique_checks
-0
+1
 '#--------------------FN_DYNVARS_005_04-------------------------#'
 SET @@session.unique_checks =1;
 SELECT @@session.unique_checks;

sql/sys_vars.ic

@@ -1704,13 +1704,16 @@ public:
     return false;
   }
   void session_save_default(THD *thd, set_var *var)
-  { var->save_result.ulonglong_value= global_var(ulonglong) & bitmask; }
+  {
+    var->save_result.ulonglong_value=
+      (reverse_semantics == !(global_var(ulonglong) & bitmask));
+  }
   void global_save_default(THD *thd, set_var *var)
   { var->save_result.ulonglong_value= option.def_value; }
 
   uchar *valptr(THD *thd, ulonglong val)
   {
-    thd->sys_var_tmp.my_bool_value= reverse_semantics ^ ((val & bitmask) != 0);
+    thd->sys_var_tmp.my_bool_value= (reverse_semantics == !(val & bitmask));
     return (uchar*) &thd->sys_var_tmp.my_bool_value;
   }
   uchar *session_value_ptr(THD *thd, const LEX_STRING *base)