added variables relay_log, relay_log_index, relay_log_info_file to init_vars[]
to make them visible within SHOW VARIABLES


mysql-test/r/rpl_flush_log_loop.result:
  test result
mysql-test/t/rpl_flush_log_loop.test:
  test case
sql/set_var.cc:
  added variables relay_log, relay_log_index, relay_log_info_file to init_vars[]
  to make them visible within SHOW VARIABLES