backporting of bug@30703 to 5.1.
The fixes are backed up with a regression test.



mysql-test/include/test_fieldsize.inc:
  waiting to stop is to be actually exclusively for SQL thread.
mysql-test/suite/rpl/r/rpl_show_slave_running.result:
  new results file is added.
mysql-test/suite/rpl/t/rpl_show_slave_running.test:
  regression test for bug#30703 is added.
sql/mysqld.cc:
  refining `show status like slave_running' handler to correspond to one of
  `show slave status'.
sql/slave.cc:
  A dbug-sync point is added to complement the regression test.