- Update comments
 - Make "write_file" fail if file already exist
 - Remove temporary files created by test cases


client/mysqltest.c:
  Improve function comments
  Make write_file fail if file already exist
mysql-test/r/mysqltest.result:
  Update test result after adding new test and updating description
  of argumements to chmod
mysql-test/t/bootstrap.test:
  Remove temporary file created by testcase
mysql-test/t/mysql.test:
  Remove temporary file created by testcase
mysql-test/t/mysqladmin.test:
  Remove temporary file created by testcase
mysql-test/t/mysqltest.test:
  Remove temporary file created by testcase 
  Add test to show that "write_file" fails if file already exist
mysql-test/t/sp-destruct.test:
  Remove temporary file created by testcase