Variable character_set_system is selectable now
  More accurate error message results during 
  update of character_set_system and some other read only variables


mysql-test/r/variables.result:
  Fix for bug#11775 Variable character_set_system does not exist (sometimes).
    test case
mysql-test/t/variables.test:
  Fix for bug#11775 Variable character_set_system does not exist (sometimes).
    test case