Added --use-threads option for porting to Windows. 


client/mysqlslap.c:
  Addinng thread option for porting to windows (second piece of code)
  Also added flush call to fix one bug
mysql-test/r/mysqlslap.result:
  New results
mysql-test/t/disabled.def:
  Bug 16167 should not be fixed
mysql-test/t/mysqlslap.test:
  Added test for new use-threads option