interrupt I/O after thr_alarm_kill()

test case for order by desc coredump


mysql-test/t/order_by.test:
  added test case for order by desc bug
sql/slave.cc:
  close socket on slave stop
sql/slave.h:
  close socket on slave stop
sql/sql_class.cc:
  close socket on slave stop
sql/sql_class.h:
  close socket on slave stop
sql/sql_parse.cc:
  changed kill_one_thread to use prepare_to_die()
sql/sql_repl.cc:
  close socket on slave stop