added support for alphanumeric variables - import MASTER_MYPORT and
    SLAVE_MYPORT from the environment
mysql-test/mysql-test-run.sh
    export MASTER_MYPORT and SLAVE_MYPORT for mysqltest
mysql-test/t/rpl000015.test
    use MASTER_MYPORT
mysql-test/t/rpl000016.test
    use MASTER_MYPORT