Try kill all children to the mysql-test-run process when exiting
mtr_process.pl:
  Added new function mtr_exit() that try kill all children to the mysql-test-run process when exiting
mysql-test-run.pl:
  Try kill all children to the mysql-test-run process when exiting
  New --vardir caused problems on Windows
  Changed faulty calls to error() to mtr_error()